U.S. patent application number 13/272547 was filed with the patent office on 2013-04-18 for wavelength monitor, wavelength lockable laser diode and method for locking emission wavelength of laser diode.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. The applicant listed for this patent is Phillip Edwards, Chie FUKUDA, Toshimitsu Kaneko, Lance Thompson. Invention is credited to Phillip Edwards, Chie FUKUDA, Toshimitsu Kaneko, Lance Thompson.
Application Number | 20130094527 13/272547 |
Document ID | / |
Family ID | 48085969 |
Filed Date | 2013-04-18 |
United States Patent
Application |
20130094527 |
Kind Code |
A1 |
FUKUDA; Chie ; et
al. |
April 18, 2013 |
WAVELENGTH MONITOR, WAVELENGTH LOCKABLE LASER DIODE AND METHOD FOR
LOCKING EMISSION WAVELENGTH OF LASER DIODE
Abstract
A wavelength monitor monolithically integrated with a tunable LD
is disclosed. The wavelength monitor includes at least two filters,
each having a periodic transmission spectrum but a period between
nearest neighbor periods is different from the other. A
transmittance of the first filter and another transmittance of the
second filter at a grid wavelength attributed to the WDM system
forms a combination which is specific to the grid wavelength bur
different from other combinations at other grid wavelengths.
Inventors: |
FUKUDA; Chie; (Yokohama-shi,
JP) ; Kaneko; Toshimitsu; (Yokohama-shi, JP) ;
Thompson; Lance; (San Jose, CA) ; Edwards;
Phillip; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FUKUDA; Chie
Kaneko; Toshimitsu
Thompson; Lance
Edwards; Phillip |
Yokohama-shi
Yokohama-shi
San Jose
San Jose |
CA
CA |
JP
JP
US
US |
|
|
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
Osaka
JP
|
Family ID: |
48085969 |
Appl. No.: |
13/272547 |
Filed: |
October 13, 2011 |
Current U.S.
Class: |
372/20 |
Current CPC
Class: |
H01S 5/0687 20130101;
H01S 5/1209 20130101; H01S 5/1212 20130101; H01S 3/13 20130101;
H01S 5/02248 20130101; H01S 5/06256 20130101 |
Class at
Publication: |
372/20 |
International
Class: |
H01S 3/10 20060101
H01S003/10 |
Claims
1. A wavelength monitor integrated with a tunable LD to form a
wavelength lockable LD, comprising: a first optical filter to
transmit light generated by the tunable LD, the first optical
filter having a first transmittance spectrum periodically varying
in a wavelength range of a WDM system; and a second optical filter
to transmit light generated by the tunable LD, the second optical
filter having a second transmittance spectrum periodically varying
in the wavelength range of the WDM system, wherein the first
transmittance spectrum and the second transmittance spectrum at a
grid wavelength of the WDM system have a combination in respective
transmittances specific to the grid wavelength different from
combinations of respective transmittances at other grid wavelengths
of the WDM system.
2. The wavelength monitor of claim 1, wherein at least one of the
first optical filter and the second optical filter has an
arrangement of a Mach-Zender filter.
3. The wavelength monitor of claim 1, wherein at least one of the
first optical filter and the second optical filter has an
arrangement of a ring filter.
4. The wavelength monitor of claim 1, wherein the transmittance of
the first transmittance spectrum at respective grid wavelengths of
the WDM system is substantially constant, wherein the transmittance
of the second transmittance spectrum at respective grid wavelengths
monotonically varies within the wavelength range of the WDM
system.
5. The wavelength monitor of claim 4, wherein the first
transmittance spectrum periodically varies by N cycles in the
wavelength range of the WDM system and the second transmittance
spectrum periodically varies by less than N cycles but greater than
N-0.5 cycles in the wavelength range of the WDM system.
6. The wavelength monitor of claim 5, wherein the second
transmittance spectrum periodically varies by less than N cycles
but greater than N-0.1 cycles in the wavelength range of the WDM
system.
7. The wavelength monitor of claim 1, wherein the wavelength
monitor further includes a base PD, a first PD, and a second PD
each of which are monolithically integrated with the first optical
filter and the second optical filter; wherein the first PD monitors
light transmitted through the first optical filter, the second PD
monitors light transmitter through the second optical filter, and
the base PD directly monitors light generated in the tunable
LD.
8. A wavelength lockable LD, comprising: a tunable LD to emit light
with an emission wavelength substantially coincident with a target
grid wavelength of a WDM system; a wavelength monitor including a
first optical filter and a second optical filter, the first optical
filter having a first transmittance spectrum periodically varying
in a wavelength range of the WDM system, the second optical filter
having a second transmittance spectrum periodically varying in the
wavelength range, the first transmittance spectrum and the second
transmittance spectrum at the target grid wavelength having a
combination in respective transmittances specific to the target
grid wavelength different from combinations in transmittances at
other grid wavelengths; a controller to tune the emission
wavelength of the tunable LD such that the first optical filter and
the second optical filter show a combination in respective
transmittances equal to the combination at the grid wavelength.
9. The wavelength lockable LD of claim 8, wherein the first optical
filter and the second optical filter has an arrangement selected
from a group of a ring filter and a Mach-Zender filter.
10. The wavelength lockable LD of claim 8, wherein the first
transmittances spectrum at respective grid wavelengths have
substantially constant transmittance in the wavelength range, and
wherein the second transmittance spectrum at respective grid
wavelengths have transmittances monotonically varying in the
wavelength range.
11. The wavelength lockable LD of claim 10, wherein the first
transmittance spectrum varies N cycles in the wavelength range, and
the second transmittance spectrum varies less than N cycles but
greater than N-0.5 cycles in the wavelength range.
12. The wavelength lockable LD of claim 11, wherein the second
transmittance spectrum varies less than N cycles but greater than
N-0.1 cycles in the wavelength range.
13. The wavelength lockable LD of claim 8, wherein the tunable LD
has an arrangement selected from a group of a Fabry-Perot LD and a
DFB LD, and wherein the controller controls a temperature of the
tunable LD to tune the emission wavelength thereof.
14. The wavelength lockable LD of claim 8, wherein the tunable LD
includes a gain region, and two SG-GBR regions putting the gain
region therebetween, and wherein the controller controls biases
each applied to the gain region and two SG-DBR regions to tune the
emission wavelength of the tunable LD.
15. The wavelength lockable LD of claim 8, wherein the tunable LD
includes a SG-DFB region and a CSG-DBR region, and wherein the
controller controls biases applied to the SG-DFB region and heater
powers supplied to heaters monolithically integrated in the CSG-DBR
region to tune the emission wavelength of the tunable LD.
16. A method to tune an emission wavelength of a wavelength
lockable LD to a target grid wavelength of a WDM system, the
wavelength lockable LD including a tunable LD and a wavelength
monitor monolithically integrated with the tunable LD, the
wavelength monitor including a first optical filter having a first
periodic transmittance spectrum and a second optical filter having
a second periodic transmittance spectrum, the first periodic
transmittance spectrum and the second periodic transmittance
spectrum having a specific combination in respective transmittances
at the target grid wavelength different from combinations in
transmittances at other grid wavelengths, the method comprising
steps of: guiding light generated by the tunable LD to the first
optical filter and the second optical filter; detecting a first
output of the first optical filter, and a second output of the
second optical filter; tuning the emission wavelength of the
tunable LD based on the first output so as to set the first output
equal to a first preset transmittance for the first periodic
transmittance spectrum; and verifying the emission wavelength of
the tunable LD by comparing the second output with a second preset
transmittance of the second periodic transmittance spectrum,
wherein the first preset transmittance and the second preset
transmittance constitute the specific combination at the target
grid wavelength.
17. The method of claim 16, further including steps of, when the
first output and the second output are offset from the specific
combination, tuning the emission wavelength based on the second
output and tuning again the emission wavelength based on the first
output again after verifying the emission wavelength.
18. The method of claim 16, wherein said step of tuning the
emission wavelength includes a step of varying a temperature of the
tunable LD to tune the emission wavelength thereof when the tunable
LD has an arrangement selected from a group of Fabry-Perot type and
DFB type.
19. The method of claim 16, wherein said step of tuning the
emission wavelength includes a step of varying biased supplied to
the tunable LD to tune the emission wavelength thereof when the
tunable LD includes a SG-DBR region.
20. The method of claim 16, wherein said step of tuning the
emission wavelength includes a step of varying heater power
supplied to heaters monolithically integrated in the tunable LD to
tune the emission wavelength thereof when the tunable LD includes a
CSG-DBR region.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a wavelength monitor, a
wavelength lockable laser diode (hereafter denoted as LD)
implementing with the wavelength locker, and a method for locking
the emission wavelength of a tunable LD.
[0003] 2. Related Background Art
[0004] Kimoto et al. disclosed a semiconductor laser module
including a tunable distributed feedback (hereafter denoted as DFB)
LD and a wavelength locker in a single package (Furukawa Technical
Report 112, July, 2003, pp. 1 to 4). The wavelength locker
disclosed therein had two photodiodes (hereafter denoted as PD),
one of which detected a portion of back facet light directly from
the DFB-LD, while the other of which detected another portion of
the back facet light through an etalon filter. The emission
wavelength of the DFB-LD may be tuned through these two
detections.
[0005] Recent wavelength division multiplexing (hereafter denoted
as WDM) system has ruled a span between nearest two grid
wavelengths as 50 GHz within the wavelength region of 192 to 197
THz, which corresponds to the 1550 nm band. In such a system, an
optical signal source is required to control the emission
wavelength further precisely and stably. The emission wavelength of
an LD often fluctuates due to operating temperature and/or a
long-term degradation of device performance. The wavelength locker
for such an LD is inevitable in the WDM system.
[0006] Conventional wavelength lockers have been implemented with
an optical component having a periodic transmission spectrum
against wavelengths. An etalon filter is one of typical components
showing such periodic transmission spectrum. The period between the
transmission maxima of the etalon filter matches with the span of
the grid wavelengths of the WDM system.
[0007] However, the wavelength locker described above leaves a
subject that the locking performance may be available only within a
narrow wavelength range. That is, when the emission wavelength of
the LD shifts more than one period of the transmission spectrum,
the wavelength locker tunes the emission wavelength next to the
target wavelength, which is a fatal subject when such a wavelength
locker is going to be applied to the recent WDM system.
[0008] Moreover, recent optical apparatus further requests to make
the housing or package thereof as compact as possible. When the
wavelength locker is realized by discrete components of the DFB-LD,
the etalon filter, and PDs; these devices are arranged
independently and coupled with, for instance, a condenser lens to
obtain a satisfactory coupling condition, which inevitably enlarges
the size of the housing/package.
SUMMARY OF THE INVENTION
[0009] An aspect of the present invention relates to a wavelength
monitor monolithically integrated with a tunable LD. The wavelength
monitor and the tunable LD may form a wavelength lockable LD. The
wavelength monitor according to the present invention includes a
first optical filter and a second optical filter, each of which may
transmit light generated by the tunable LD and have the
transmission spectrum periodically varying in a wavelength range
attributed to the WDM system. A feature of the wavelength monitor
of the present invention is that two transmission spectra have a
combination in respective transmittances which is specific to the
grid wavelength of the WDM system but different from combinations
of respective transmittance at other grid wavelengths.
[0010] Another aspect of the present invention relates to a
wavelength lockable LD that includes a tunable LD, a wavelength
monitor monolithically integrated with the tunable LD, and a
controller. The tunable LD may emit light with an emission
wavelength substantially coincident with a target grid wavelength
of the WDM system. The wavelength monitor may include first and
second optical filters, each having the transmission spectrum
periodically varying in the wavelength range of the WDM system. A
feature of the present wavelength lockable LD is that two
transmission spectra have a combination of transmittances thereof
which is specific to the target grid wavelength but different from
combinations in the transmittances at other grid wavelengths. The
controller may tune the emission wavelength of the tunable LD such
that two optical filters show a combination in respective
transmittances equal to the combination at the grid wavelength.
[0011] Still another aspect of the present invention relates to a
method to tune the emission wavelength of the wavelength lockable
LD to a target wavelength. The wavelength lockable LD
monolithically integrates a tunable LD with a wavelength monitor.
The wavelength monitor includes first and second optical filters
each having a periodic transmission spectrum whose transmittances
at the target wavelength constitutes a combination which is
specific to the target wavelength but different from combinations
of transmittances at other wavelengths. The method may include
steps of: (a) guiding light generated by the tunable LD to the
first and second optical filters; (b) detecting respective outputs
of the first and second optical filters; (c) tuning the emission
wavelength of the tunable LD based on the output of the first
optical filter so as to set the output of the first optical filter
equal to a first preset transmittance for the transmission spectrum
of the first optical filter; and (d) verifying the emission
wavelength by comparing the output of the second optical filter
with a second preset transmittance for the transmission spectrum of
the second optical filter. A feature of the present method is that
the first preset transmittance and the second preset transmittance
constitute the specific combination of the transmittances at the
target grid wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0013] FIG. 1 is a plan view showing a wavelength lockable LD
according to the first embodiment of the present invention;
[0014] FIG. 2 magnifies the wavelength monitor shown in FIG. 1;
[0015] FIG. 3 shows a block diagram of a control circuit to tune
the emission wavelength of the wavelength lockable LD shown in FIG.
1;
[0016] FIG. 4 shows a flow chart to tune the emission wavelength to
a target wavelength;
[0017] FIGS. 5A to 5C show transmission spectra of the ring filters
implemented within the wavelength monitor shown in FIG. 2;
[0018] FIG. 6 is a plan view showing another wavelength lockable LD
according to the second embodiment of the invention;
[0019] FIG. 7 is a plan view showing still another wavelength
lockable LD according to the third embodiment of the invention,
where the wavelength lockable LD of the present embodiment
implements with Mach-Zender filters in the wavelength monitor;
[0020] FIG. 8 magnifies the wavelength monitor implemented within
the wavelength lockable LD shown in FIG. 7;
[0021] FIGS. 9A to 9C show transmission spectra of the Mach-Zender
filters implemented within the wavelength monitor shown in FIG.
8;
[0022] FIG. 10 is a plan view showing still another wavelength
lockable LD according to the fourth embodiment of the
invention;
[0023] FIG. 11 shows a cross section of another tunable LD able to
be integratable with the wavelength monitor of the present
invention;
[0024] FIG. 12 is a plan view showing still another wavelength
lockable LD according to the fifth embodiment of the invention,
where the wavelength lockable LD integrates the tunable LD shown in
FIG. 11;
[0025] FIG. 13 shows a block diagram of the circuit to tune the
emission wavelength of the wavelength lockable LD shown in FIG. 12;
and
[0026] FIG. 14 shows a cross section of still another tunable LD
integratable with the wavelength monitor according to the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0027] Next, some preferred embodiments according to the present
invention will be described in detail. In the description of the
drawings, the same numerals or symbols will refer to the same
elements if possible without overlapping explanations. Aspect ratio
in respective drawings does not always reflect the practical
dimensions, and sometimes modified by the explanation sake.
First Embodiment
[0028] A wavelength lockable LD according to the first embodiment
of the present invention will be described first. FIG. 1 is a plan
view showing the wavelength lockable LD 1A according to the first
embodiment of the invention.
[0029] The wavelength lockable LD 1A includes a tunable LD 3, a
wavelength monitor 5, a semiconductor optical amplifier (hereafter
denoted as SOA) 7, and an optical modulator 9.
[0030] First, the wavelength monitor 5 will be described. FIG. 2
magnifies a primary portion of the wavelength monitor 5. The
wavelength monitor 5 includes the base PD 60, the first PD 61, the
second PD 62, and some waveguides 59 which includes three
waveguides, 50-52. The base waveguide 50 has two waveguides, 50a
and 50b, putting the base PD 60 therebetween. One of the base
waveguides 50a, which is the front waveguide, optically couples
with the active waveguide 311 in the tunable LD 3; while, the other
waveguide 50b, the rear waveguide, optically couples with the
optical coupler 69. The optical coupler 69 is a type of the
multi-mode interference (MMI) coupler that divides an optical beam
incoming from the rear waveguide 50b into two beams each
propagating in the first and second waveguides, 51 and 52.
[0031] The first waveguide 51 includes a linear waveguide 51L
extending horizontally in FIG. 2 and a ring filter 51F that
comprises a pair of liner waveguides 51R1 and a pair of
semicircular waveguides 51R2, where these waveguides, 51R1 and
51R2, form an oval and one of the liner waveguides 51R1 couples
with the liner waveguide 51L. The first waveguide 51 has a periodic
transmission spectrum, details of which depend on the optical path
length of the first ring filter 51F. That is, adjusting the
refractive index of the ring filter 51F and the dimensions thereof,
the period of the periodic transmission spectrum of the first
waveguide 51 may be controlled.
[0032] The second waveguide 52 also includes another ring filter
52F comprising a pair of linear waveguides 52R1 and a pair of
semicircular waveguides 52R2, where those waveguides form another
oval. One of the linear waveguides 52R1 couples with the linear
waveguide 52L that propagates one of optical beams divided by the
MMI coupler 69. The period of the periodic transmission spectrum of
the second waveguide 52 may be adjusted by setting the optical path
length of the second ring filter 52F. In the present embodiment
shown in FIG. 2, the optical path length of the second ring filter
52F is slightly different from that of the first ring filter 51F.
Details of the optical path lengths and differences of the periodic
transmittance of respective ring filters, 51F and 52F, will be
described later.
[0033] The first PD 61 receives light transmitting through the
first waveguide 51, while, the second PD 62 receives light
transmitting through the second waveguide 52.
[0034] The waveguides 59 in the wavelength monitor 5 propagates a
portion of the light generated in the tunable LD 3 to respective
PDs, 60 to 62, and detected thereby. Specifically, the front
waveguide 50a first transmits a portion of the light generated in
the tunable LD 3 to the base PD 60. The PD may detect a portion of
thus transmitted light and pass a rest portion of the light. The
rear waveguide 50b may carry this rest portion of the light to the
MMI coupler 69. The MMI coupler 69 may divide this rest portion of
the light into two parts, one of which propagates on the first
waveguide 51, modulated by the first ring filter 51F, and detected
by the first PD 61; while, the other part propagates on the second
waveguide 52, modulated by the second ring filter 52F, and detected
by the second PD 62.
[0035] Next, details of the optical modulator 9 will be described.
The optical modulator 9 according to the present invention includes
a pair of waveguides 91 extending in parallel to each other between
two MMI couplers, 81 and 82, with the arrangement of 2.times.2.
That is, two modulation waveguides 91 are divided by the MMI
coupler 81 which optically couples with the active waveguide 311
through the SOA 7, while, combined by the other MMI coupler 82
which optically couples with the output port 1P of the wavelength
lockable LD 1A. The later MMI coupler 82 also couples with a
surplus waveguide 98, where it is terminated in a side of the
tunable LD 3. The light, generated in the tunable LD 3 and
amplified by the SOA 7, is divided into two parts by the first MMI
coupler 81. The divided beams each propagates in respective
modulation waveguides 91 and merges in the second MMI coupler 82 to
output from the port 1P. During the propagation in respective
modulation waveguides 91, each beam senses the electric field
different from the other, which may cause a phase difference
between them; the light combined by the second MMI coupler 82 may
be modulated. Specifically, applying an electrical modulation
signal between two electrodes 95 in the modulation waveguides 91,
the light propagating in one of the modulation waveguides 91
advances or delays the phase thereof by n with respect to the light
propagating in the other modulation waveguide 91; accordingly, the
merged light substantially vanishes when the phase difference
between two beams is .pi. but receives no effect when the phase of
respective beams coincides to the other. Thus, the light emitted
from the tunable LD 3 may be modulated by the modulation signal
applied to the electrodes 95. The electrodes 96 which are also
arranged in the modulation waveguides 91 may preset the phase
difference between two beams.
[0036] Next, a method to control the wavelength lockable LD
specifically, a method to tune the emission wavelength of the
tunable LD 3, according to an embodiment of the present invention,
will be described. The emission wavelength of the wavelength
lockable LD 1A may be tuned so as to coincide with one of grid
wavelengths of the WDM system. In the explanation below, this one
of grid wavelengths is called as the target wavelength.
[0037] FIG. 3 is a block diagram of the control system for the
wavelength lockable LD 1A, where the control system primarily
includes a micro control unit (MCU) 121. The MCU 121, by receiving
tree outputs each coming from the base PD 60, and the first and
second PDs, 61 and 62, may control the tunable LD 3, exactly, the
gain region 31 thereof, the thermo-electric controller (hereafter
denoted as TEC) 2, the SOA 7 and the modulator 9 so as to tune the
emission wavelength of the tunable LD 3 coincident with the target
wavelength. The MCU 121 may output control signals, V.sub.31,
V.sub.7, V.sub.9 and V.sub.2, to respective elements.
[0038] Next, an algorithm to tune the emission wavelength of the
wavelength lockable LD 1A according to the present embodiment will
be described. FIG. 4 is a flow chart of the algorithm to tune the
emission wavelength. The method according to the present embodiment
includes steps of: [0039] (S1) preparing a tunable LD 3 and
measuring initial conditions for the tuning; [0040] (S3) guiding
light generated in the tunable LD 3 to respective PDs, 60 to 62,
and detecting it by respective PDs in the wavelength monitor 5;
[0041] (S5) tuning the emission wavelength based on the outputs of
the base and the first PDs, 60 and 61; [0042] (S7) verifying the
emission wavelength to be coincide with the target wavelength based
on the base and second PDs, 60 and 62; and [0043] (S9) when the
emission wavelength is different from the target wavelength, tuning
the emission wavelength based on the outputs of the base and the
second PDs, 60 and 61, again.
S1 Pre-process
[0044] The step S1 prepares a wavelength lockable LD 1A and initial
operating conditions of the wavelength lockable LD 1A.
Specifically, step S1 first sets the wavelength lockable LD 1A in a
preset temperature by controlling the TEC 2; then monitors the
emission wavelength of the LD 3 by an external wavelength detector
under a condition where the gain region 31 of the tunable LD 3 is
biased by the signal V.sub.31 through the electrode 315 thereof.
Next, shifting the temperature of the lockable LD 1A by controlling
the TEC 2 such that the emission wavelength of the LD 3 coincides
with one of grid wavelength of WDM communication system, which will
be called as the target wavelength, parameters listed below are
recorded when the emission wavelength just coincides with the
target wavelength. Parameters to be recorded are the transmittance
of the first ring filter 51F, that of the second ring filter 52F,
the temperature of the TEC 2, the bias V.sub.31 for the gain region
31, and the bias V.sub.7 for the SOA 7. Two transmittances may be
calculated from the outputs of three PDs, 60 to 61. The initial
conditions described above are measured for respective grid
wavelengths of the WDM system, and saved in, for instance, a
look-up-table prepared in the MCU 121.
S3 Guiding Light
[0045] Step S3 selects one of grid wavelengths, which is the target
wavelength, and sets parameters of the temperature of the TEC 2,
the bias V.sub.31 and the bias V.sub.7 in respective devices to
generate light by the tunable LD 3. In this process, the bias
V.sub.7 applied to the SOA 7 and the bias V.sub.9 applied to the
modulator 9 are preferably set such that the light emitted from the
output port 1P substantially vanishes.
[0046] A portion of light generated by the tunable LD 3 enters
respective PDs, 60 to 62, through the waveguides 59 and detected
thereby. The first PD 61 may detect light filtered by the first
ring filter 51F, while, the second PD 62 may detect light filtered
by the second ring filter 52F.
S5 First Tuning
[0047] Step S5 tunes the emission wavelength based on the output of
the first and second PDs, 61 and 62. The transmission spectra of
the first and second ring filters, 51F and 52F, will be explained
in advance to describe steps S5.
[0048] FIGS. 5A to 5C show the transmission spectra of two ring
filters, 51F and 52F, where the horizontal axis shows the
wavelength and the vertical axis corresponds to the transmittance.
Bold lines in these figures are the transmission spectrum for the
first ring filter 51F, while, thin lines are those of the second
ring filter 52F. The grid wavelengths of the WDM system, which are
denoted by the mark "G" in the figures, are arranged with a
constant span of 50 GHz (about 0.4 nm)
[0049] As explicitly shown in the figures, especially, in FIGS. 5B
and 5C, the period of the transmission spectrum of the first ring
filter 51F is different from the period of the transmission
spectrum of the second ring filter 52F. In the present embodiment,
the latter period for the second ring filter 52F is slightly
greater than that of the former period for the first ring filter
51F.
[0050] Moreover, the period of the transmission spectrum for the
first ring filter 51F is substantially equal to the span of the
grid wavelengths of the WDM system. Accordingly, the transmittance
of the first ring filter 51F at the grid wavelengths, which is
denoted by filled circles, becomes substantially constant to be
about 0.817.
[0051] In a wavelength region where the tunable LD 3 may emit
light, which is 1520 to 1570 nm in the present embodiment, the
first ring filter 51F and the second ring filter 52F satisfy the
following conditions: the transmission spectrum of the first ring
filter 51F shows N cycles, where N is an integer equal to or
greater than 2, and the transmission spectrum of the second ring
filter 52F shows less than N cycles but equal to or greater than
N-0.5 cycles, preferably, the second ring filter 52F shows less
than N-0.1 cycles but equal to or greater than N-0.5 cycles. In the
present embodiment, N is 125, that is, the transmission spectrum of
the first ring filter 51F shows 125 cycles in the wavelength range
of 1520-1570 nm, while, the second ring filter 52F shows about
124.5 cycles in the same wavelength range, which is just less than
that of the first ring filter 51F.
[0052] Moreover, as shown in FIGS. 5A to 5C, the transmittance of
the second ring filter 52f at the grid wavelengths G, which are
illustrated by open squares, monotonically varies from the shortest
grid wavelength to the longest grid wavelength, where the
transmittance monotonically decreases in the present embodiment,
which may give a combination of transmittances which is specific to
the target wavelength bur different from combinations in other grid
wavelengths.
[0053] The transmission spectrum of the first ring filter 51F
described above is available in an arrangement thereof where a
length of the linear waveguide 51R1 is 496 .mu.m, a diameter of the
semi-circular waveguide 51R1 is 50 .mu.m, and a material thereof
has refractive index of 3.83005 at the wavelength of 1520 nm.
While, the periodic transmission spectrum for the second ring
filter 52F is available in an arrangement where a length of the
linear waveguide 52R1 is 495 .mu.m, a diameter of the semi-circular
waveguide is 50 .mu.m and a material thereof has the refractive
index same with that of the first ring filter 51F.
[0054] The first tuning process S5 tunes the emission wavelength of
the tunable LD 3 based on the output of the base PD 60 and that of
the first PD 61. For instance, one case will be explained where the
emission wavelength of the tunable LD 3 is put close to or
substantially equal to one of grid wavelengths, which is marked by
"GP1" in FIG. 5B. Referring to the lock-up table above described,
the MCU 121 adjusts the temperature of the TEC 2 by sending the
signal V2 to the TEC 2 such that the transmittance of the first
ring filter 51F obtained by a ratio of the output of the first PD
61 against the output of the base PD 60 becomes equal to a value at
the grid wavelength GP1, which is about 0.817 in FIG. 5B.
[0055] The process of the first tuning S5 is thus carried out.
However, the transmittance of the first ring filter 51F shows the
periodic characteristic whose period is substantially equal to the
span of the grid wavelengths G as described above; accordingly, the
MCU 121 may occasionally tune the emission wavelength to another
grid wavelength GP2 shown in FIG. 5C when the emission wavelength
fluctuates more than one period of the transmission spectrum of the
first ring filter 51F, which is 0.4 nm in the present embodiment.
In other words, the first ring filter 51F may provide a plurality
of equivalent wavelengths, and the MCU 121 may operate to set the
emission wavelength in one of the equivalent wavelengths, which is
sometimes different from the target wavelength GP1.
S7 Verification
[0056] The verifying process S7 may be carried out after the first
tuning S5. As described above, even if the first tuning S5 sets the
emission wavelength in another grid wavelength different from the
target wavelength, the verification process S7 may detect the fact
that the emission wavelength is not the target wavelength by the
outputs of the second PD 62 and the base PD 60.
[0057] Specifically, the transmittance of the second ring filter
52F at the grid wavelengths G are different from each other as
shown in FIGS. 5A to 5C in the wavelength range. Accordingly, the
transmittance of the second ring filter 52F obtained by the second
PD 62 and that of the base PD 60 when the emission wavelength is
set in another grid wavelength GP2 is shifted from the
transmittance thereof when the emission wavelength is just set in
the target wavelength GP1. For instance, the transmittance of the
second ring filter 52F, when the emission wavelength is improperly
set in the other wavelength GP2, becomes about 0.220, while, the
transmittance thereof when the emission wavelength is properly set
to the target wavelength GP1 is about 0.595.
[0058] Accordingly, the MCU 121 may verify, based on the
transmittance of the second ring filter 52F, whether the emission
wavelength is properly set to the target wavelength GP1 among the
grid wavelengths G. When the MCU 121 decides that the emission
wavelength is properly controlled to the target wavelength GP1, the
process to control the emission wavelength of the tunable LD 3 is
completed. However, the MCU 121 decides that the emission
wavelength is improperly controlled; the MCU 121 may proceed to the
second tuning S9.
S9 Second Tuning
[0059] The second tuning S9 controls the emission wavelength first
by the output of the second PD 62 and that of the base PD 60; then
by the output of the first PD 61 and that of the base PD 60.
Specifically, the MCU 121 first controls the TEC 2 such that the
transmittance of the second ring filter 52F obtained by the ratio
of the output of the second PD 62 against that of the base PD 60
puts close to the value of the transmittance thereof at the target
wavelength GP1, which is about 0.595 in the present embodiment.
After the adjustment above, the emission wavelength is fallen
within a wavelength range of one period of the transmission
spectrum in which the target wavelength GP1 belongs.
[0060] Then, the MCU 121 precisely controls the TEC 2 such that the
transmittance of the first ring filter 51F calculated by the output
of the first PD 61 and that of the base PD 60 becomes the value at
the target wavelength GP1, which is 0.817 in the present
embodiment. Because the emission wavelength of the tunable LD 3 is
set in the wavelength close to the target wavelength GP1 in advance
to the precise tuning, the MCU 121 may properly tune the emission
wavelength of the tunable LD 3 in the target wavelength GP1.
[0061] After the tuning of the emission wavelength, the MCU 121 may
further adjust the emission magnitude of the tunable LD 3 so as to
be equal to a preset magnitude by control the bias V.sub.7 applied
to the SOA 7. Furthermore, the MCU 121 may adjust the bias V.sub.9
applied to the modulator 9 so as to get a properly modulated light
from the modulator 9. Thus, the light having the target wavelength
which is precisely adjusted, the preset magnitude, and the properly
modulated may be obtained from the output port 1P.
Second Embodiment
[0062] Next, the second embodiment of the wavelength lockable LD
according to the present invention will be described in detail.
FIG. 6 is a plan view of the wavelength lockable LD 1B according to
the second embodiment of the invention. The wavelength lockable LD
1B has a different arrangement from those of the first wavelength
lockable LD 1A in viewpoints of the SOA-less tunable LD 3 and a
different wavelength monitor 5B.
[0063] The wavelength monitor 5B of the present embodiment exists
in downstream of the modulator 9; that is, the wavelength monitor
5B couples in the first waveguide 51 thereof with the end of the
active waveguide 311 of the tunable LD 3 through the surplus
waveguide 98, the modulation waveguide 91, and the MMI coupler 81
of the modulator 9. The other end of the active waveguide 311 is
terminated in the end facet of the wavelength lockable LD 18.
[0064] The wavelength monitor 5B of the present embodiment also
includes the base PD 160, the first PD 161, and the second PD 162.
The light entering the waveguide 150 divided by the MMI coupler 82
is partially absorbed by the base PD 160 but a most part of the
light passes the base PD 160 and divided into two beams, one of
which enters the first ring filter 151F, while, the other of which
enters the second ring filter 152F. Both light beams entering
respective waveguides, 151 and 152, are terminated after passing
the first PD 161 and the second PD 162.
[0065] The wavelength monitor 5B may tune the emission wavelength
of the tunable LD 3 even the emission wavelength thereof remarkably
shift from the target wavelength by the mechanism same as those of
the wavelength monitor 5A according to the first embodiment.
Third Embodiment
[0066] FIG. 7 is a plan view of a wavelength lockable LD 1C
according to the third embodiment of the present invention. The
wavelength lockable LD 1C includes the tunable LD 3, the SOA 7, the
modulate 9, and another type of wavelength monitor 5C.
[0067] The wavelength monitor 5C according to the present
embodiment will be described in detail. FIG. 8 is a plan view of
the wavelength monitor 5C. The wavelength monitor 5C includes three
PDs, namely, the base PD 260, the first PD 261, and the second PD
262, and the waveguides 259 that comprises the primary waveguide
250, and the first and second waveguides 251 and 252, each divided
from the primary waveguide 250. The primary waveguide 250 includes
the front waveguide 250b arranged in upstream of the base PD 260
and the rear waveguide 250a disposed in downstream of the base PD
260.
[0068] The front waveguide 250b optically couples with the active
waveguide 311 of the tunable LD 3 as illustrated in FIG. 7. The
base PD 250, which is put between the front waveguide 250b and the
rear waveguide 250a, may receive raw light of the tunable LD 3,
where the raw light means that the light directly comes from the
tunable LD 3 without passing any optical filters.
[0069] The rear waveguide 250a couples with the coupler 269 with
the arrangement of 1.times.2 multi-mode coupler that divides light
passing through the base PD 260 into two beams, one of which enters
the first waveguide 251, while, the other of which enters the
second waveguide 252.
[0070] One feature of the wavelength monitor 5C according to the
present embodiment is that the first waveguide 251 has an
arrangement of the Mach-Zender filter 251F. Specifically, the first
waveguide 251 is divided into two waveguides, 251R1 and 251R2, then
these two waveguides, 251R1 and 251R2, merge again to constitute
the Mach-Zender filter 251F. But the optical path length of two
waveguides from the branch to the merger is different from the
other.
[0071] The transmission spectrum of the Mach-Zender filter 251F
shows the periodic behavior with respect to the wavelength of the
light passing therethrough. A difference of the physical dimensions
of two waveguides from the branch to the merger and the refractive
index of material constituting two branches may influence the
period of the periodic transmission spectrum.
[0072] The second Mach-Zender filter 252F has the mechanism same
with those of the first Mach-Zender filter 251F above described.
However, the difference of the optical path length between two
waveguides from the branch to the merger is different from the
other Mach-Zender filter.
[0073] Specifically, the period of the transmission spectrum of the
first Mach-Zender filter 251F is different from the period of the
transmission spectrum of the second Mach-Zender filter 252F.
[0074] As illustrated in FIG. 8, the first Mach-Zender filter 251F
is put between the base PD 260 and the first PD 261; that is, the
light output from the base PD 260 and transmitted through the first
Mach-Zender filter 251F enters the first PD 261. While, the second
Mach-Zender filter 252F is put between the base PD 260 and the
second PD 262. The light coming from the base PD 260 and passing
through the second Mach-Zender filter 252F enters the second PD
262.
[0075] The transmission spectra of the Mach-Zender filters, 251F
and 252F, and an algorithm to decide the current emission
wavelength of the tunable LD 3 will be described in detail.
[0076] FIGS. 9A to 9C show transmission spectra of respective
Mach-Zender filters, 251F and 252F, in a wavelength range from 1525
to 1570 nm which is attributed to the WDM system. FIGS. 9A to 9C
also denote the grid wavelengths G defined in the WDM system, where
a span between the nearest grids is set to be 50 GHz, or about 0.4
nm.
[0077] The transmission spectra, 251T and 252T, of the Mach-Zender
filters, 251F and 252F, are different from those of the ring
filters, 51F and 52F, shown in FIGS. 5A to 5C, that is, although
the transmission spectra, 251T and 252T, show a periodic behavior
but the shape thereof is sinusoidal compared with those of the ring
filter, 51F and 52F. Similar to two ring filters, 51F and 52F, the
period of the transmission spectra, 251F and 252F, are different
from the other.
[0078] The period of the first Mach-Zender filter 51F is preferably
different from the span of the grid wavelengths. Moreover, the
period of the second Mach-Zender filter 52F is preferably different
from the span of the grid wavelengths and also from the period of
the first Mach-Zender filter 51F. In FIGS. 9A to 9C, filled circles
each corresponds to the transmittance of the first Mach-Zender
filter 251F at respective grid wavelengths, while, filled squares
each corresponds to the transmittance of the second Mach-Zender
filter at respective grid wavelengths.
[0079] The peak wavelengths of the transmission spectrum of the
first Mach-Zender filter, and those of the second Mach-Zender
filter depend of the refractive index of materials of two
waveguides, 251R1 and 251R2, and the difference of the optical path
length between two waveguides, 251R1 and 251R2. The same situation
may be applicable to the second Mach-Zender filter 252F.
[0080] The MCU 121 first selects one of two Mach-Zender filters,
251F and 252F, based on a condition that which slopes in the
transmission spectrum becomes abrupt at the target wavelength.
Then, the MCU 121 receives the output from the PD that couples with
the selected Mach-Zender filter, 251F or 252F.
[0081] In an example, assuming the target wavelength is given by
GP1 in FIG. 9B, the transmission spectrum of the first Mach-Zender
filter 251T shows a greater slope, or the rate of change in the
transmission spectrum for the second Mach-Zender filter 251F
becomes larger compared with that of the other transmission
spectrum. Then, the MCU 121 selects the first Mach-Zender filter
251F and the first PD 261 for the wavelength tuning.
[0082] In another example, when the target wavelength is given by
GP2, the rate of change in the first transmission spectrum 251T for
the first Mach-Zender filter 251 becomes substantially zero, while,
that of the second Mach-Zender filter 252 becomes substantially
maximum. Then the MCU 121 may select the second filter 252F and
second PD 62 for tuning the emission wavelength of the tunable LD
3.
[0083] Then, the MCU 121 may adjust the temperature of the tunable
LD 3 such that the output of the first PD 261 with respect to the
output of the base PD 260 becomes substantially equal to the
transmittance of the first Mach-Zender filter 251F, where it seems
to be about 0.266 in the present case.
[0084] Similarly, when the emission wavelength of the LD 3 is
selected to be the other grid wavelength GP2 in FIG. 9B, the MCU
121 may control the temperature of the tunable LD 3 such that the
transmittance of the second Mach-Zender filter 252F, which may be
obtained from the output of the second PD 262 with respect to the
output of the base PD 260, becomes close to a value of 0.305.
[0085] After roughly setting the emission wavelength of the tunable
LD 3, the bias V.sub.7 applied to the SOA 7 is set so as to
increase the optical absorption by the SOA 7 not to output the
light from the output port 1P. Moreover, when the optical modulator
9 controls the output thereof, the MCU 121 sets the bias V.sub.7 to
make the SOA 7 active and the bias V.sub.9 to the modulator 9 to
modulate light from the SOA 7. Thus, the wavelength lockable LD 1C
may output the modulated light from the output port 1P.
[0086] The wavelength lockable LD 1C may monitor the wavelength of
the light generated in the tunable LD 3 by the base PD 260, the
first and second PDs, 261 and 262. The first PD 261 may monitor the
light passing through the first Mach-Zender filter 251F, while, the
second PD 262 may monitor the light passing through the second
Mach-Zender filter 252F. The transmission spectra of two
Mach-Zender filters, 251F and 252F, are different from each other;
accordingly, the wavelength monitor 5C may monitor the wavelength
in a wide range. Still further, the transmission spectra of two
Mach-Zender filters, 251F and 252F, may be set in a combination
which is specific to the grid wavelengths within the wavelength
range. Therefore, even the Mach-Zender filter shows a periodic
transmission spectrum, which means that a plurality of equivalent
wavelengths gives the substantially same transmittance, only one
wavelength specific to the combination of two transmittance may be
selected.
[0087] Thus, the wavelength lockable LD 1C may tune the emission
wavelength thereof precisely in the preset grid wavelength by the
algorithm below.
[0088] That is, one of two Mach-Zender filters, 251F and 252F, is
first selected under a condition that the slope of the
transmittance thereof against the wavelength is larger in the
requested grid wavelength, such as GP1 and GP2, which means that
one of the PDs, 261 and 262, is selected to monitor the wavelength.
Next, the emission wavelength of the tunable LD 3 is controlled
such that the transmittance of the selected Mach-Zender filter,
251F or 252F, calculated by the output of the selected PD, 261 or
262, and the base PD 260 becomes equal to the designed
transmittance of the selected Mach-Zender filter, 251F or 252F, by
controlling the temperature of the tunable LD 3. In this process,
because the selected Mach-Zender filter, 251F or 252F, shows a
greater rate of the change in the transmittance compared with that
of the unselected Mach-Zender filter, 251F or 252F, the precise
monitoring of the emission wavelength may be performed, which means
that the precise tuning of the emission wavelength may be
realized.
[0089] The wavelength lockable LD 1C of the present embodiment
couples the optical output port 1P with the end of the active
waveguide 311 opposite to the end with which the wavelength monitor
5C couples. This arrangement enables to tune the emission
wavelength of the tunable LD 3 in the desired grid wavelength under
a condition where the output port 1P emits no light.
[0090] Moreover, the wavelength monitor 5C of the present
embodiment may have two Mach-Zender filter, 251F and 252F, showing
respective transmittances in a combination thereof specific to the
grid wavelength in the preset wavelength range. Accordingly, even
the Mach-Zender filter shows the periodic transmission spectrum,
which inevitably has a plurality of equivalent wavelengths each
showing the same transmittance, the wavelength monitor 5C of the
present embodiment may determine the specific wavelength in the
wavelength range. Thus, even the tunable LD 3 shifts the emission
wavelength more than one period of the periodic transmission
spectrum of the Mach-Zender filter, 251F or 252F, the wavelength
monitor 5C may securely detect the shift and the wavelength
lockable LD 1C may recover the target emission wavelength.
Fourth Embodiment
[0091] Next, another wavelength lockable LD 1D according to the
fourth embodiment of the present invention will be described. FIG.
10 is a plan view of the wavelength lockable LD 1D of the present
embodiment, where the wavelength lockable LD 1D has substantially
same arrangement with those of the previous device 1C shown in FIG.
7 except for the arrangement of the wavelength monitor 5D.
[0092] The wavelength monitor 5D of the present embodiment
optically couples with the tunable LD 3 in the front end of the
active waveguide 311 not in the rear end thereof as those of the
previous wavelength monitor 5C. Specifically, the waveguide 350 of
the wavelength monitor 5D optically couples with the active
waveguide 311 through the MMI coupler 82, the waveguide 91, another
MMI coupler 81, where they are in the optical modulator 9, and SOA
7. The rear end of the active waveguide 311 terminates at the facet
perpendicular to the facet, 1E1 or 1E2.
[0093] The wavelength monitor 5D of the present embodiment has the
waveguides, 351 and 352, extending along the direction of the
active waveguide 311, which is perpendicular to the direction along
which the waveguide of the optical modulator 9 extends. Two
waveguides, 351 and 352, optically couple with respective
Mach-Zender filters, 351F and 352F, and the PDs, 361 and 362.
[0094] The wavelength lockable LD 1D, similar to the previous LD
1C, may keep the emission wavelength thereof in the target gird
wavelength by the mechanism same as those of the previous LD 1C,
even when the LD 1D shifts the emission wavelength thereof broadly
by some reasons. Moreover, because the active waveguide 311 and the
waveguides, 351 and 352, in the wavelength monitor 5D extend along
the direction perpendicular to the direction along which the
waveguide 91 of the optical modulator extends, the total length of
the wavelength lockable LD 1D may be decreased, which may
facilitate the assembly of the LD 1D compared with those of the
previous embodiment, where the plane size thereof is an extended
rectangle.
[0095] The wavelength monitor of the present invention may have
various alternatives. For instance, although the wavelength
monitors, 5A to 5D, has two waveguides, 50a and 50b, putting the
base PD 60 therebetween, the wavelength monitor 5A may have another
arrangement where the waveguide 50 is divided into two waveguides,
one of which couples with the base PD 60, while, the other of which
is further divided into two waveguides each coupled with the first
and second PDs, 61 and 62.
[0096] The wavelength monitor 5C in the third embodiment couples
with the rear end of the active waveguide, that is, optically
couples with the end opposite to the one coupling with the SOA 7,
the wavelength monitor 9, and the output port 1P. However, the
wavelength monitor 5C may couple with the front end of the active
waveguide 311 as those of the fourth embodiment 5D through the
optical modulator 9 and the SOA 7. On the other hand, the
wavelength monitor 5D of the force embodiment may optically couple
with the rear end of the active waveguide 311.
Fifth Embodiment
[0097] Finally, a detail of the tunable LD 3 will be further
described. FIG. 11 shows a cross section of the tunable LD 3A along
the optical axis thereof according to the present invention, while,
FIG. 12 is a plan view of a wavelength lockable LD 1E implemented
with the tunable LD 3A.
[0098] The tunable LD 3A of the present embodiment includes the
gain region 531, the phase adjusting region 533, the first and
second sampled grating distributed Bragg reflector (hereafter
denoted as SG-DBR) regions, 535 and 537. The gain region 531 has
the same arrangement with those 311 in the former embodiments.
These four regions, 531 to 537, are arranged on the common
semiconductor substrate 511, and along the optical axis of the
tunable LD 3A.
[0099] The gain region 531 has the active waveguide 531a, the upper
cladding layer 512 above the active waveguide 531a, a contact layer
531c above the upper cladding layer 512, and the anode 531d above
the contact layer 531d. The active waveguide 531a may be made of
material with a longer bandgap wavelength, equivalently, a smaller
bandgap energy, and extends along the optical axis with a length
of, for instance, 500 .mu.m. The active waveguide 311 may include a
lower separate confinement hetero-structure layer (hereafter
denoted as SCH layer), an upper SCH layer, and an active layer put
between these two SCH layers. The SCH layers may be made of GaInAsP
with a bandgap wavelength of 1.25 .mu.m and a thickness of 50
nm.
[0100] The active layer in the active waveguide 531a may have the
multiple quantum well (hereafter denoted as MQW) structure
including a plurality of well layers made of GaInAsP with a
thickness of 5 nm and a plurality of barrier layers made of also
GaInAsP with a thickness of 10 nm but having a composition
different from the composition of the well layer. The well layers
and the barrier layers are alternately stacked to each other. The
peak wavelength of the active layer measured by the
photoluminescence spectrum is 1550 nm. The total thickness of the
active waveguide 531a is, for example, about 0.2 .mu.m.
[0101] The upper cladding layer 512 may be made of p-type
semiconductor material, for instance p-type InP, with a thickness
of about 1500 nm. The upper cladding layer 512 is common in all
regions of the gain region 531, the phase adjusting region 533, the
first SG-DBR region 535 and the second SG-DBR region 537.
[0102] The contact layer 531c may be made of p-type semiconductor
material such as p-type InGaAs with a thickness of about 200 nm.
The anode electrode 531d may be made of eutectic metal such as AuZn
to make an ohmic contact to the contact layer 531c. The anode
electrode 531d and the cathode electrode 515 in the back surface of
the substrate 511 may inject carriers into the active waveguide
531a which may induce the recombination of electrons and holes in
the active waveguide 531a and photons are generated therein. The
photon may be converted into the laser light by propagating within
the active waveguide 531a.
[0103] The phase adjusting region 533, which locates outside of the
gain region 531, exactly between the gain region 531 and the first
SG-DBR region 535. The phase adjusting region 533 includes a
portion of the waveguide 535a, the contact layer 533c and the anode
electrode 533d. The waveguide 535a may be made of material having a
bandgap wavelength shorter than that of the active waveguide 531a,
namely, the bandgap energy of the waveguide in the phase adjusting
region 533 is wider than that of the active waveguide 531a, which
means that the waveguide 535a is substantially transparent for the
light generated in the gain region 531. The phase adjusting region
has a length of about 200 .mu.m along the optical axis, and may be
made of un-doped InGaAsP with the photoluminescence peak at 1350 nm
and a thickness of about 0.35 .mu.m.
[0104] The contact layer 533c and the anode electrode 533d of the
phase adjusting region 533 may be made of materials same with those
of the gain region, 531c and 531d, and may have a thickness same
with those of the gain region, respectively. The phase adjusting
region 533 has a function to adjust phase of the light propagating
in the waveguide thereof. Specifically, injecting carriers into the
waveguide 535a from the anode 533d and the cathode electrode 515,
the refractive index of the waveguide 535a may vary, which also
varies the phase propagating therein. Thus, the side mode
suppression ratio of the light emitted from the tunable LD 3 may be
enhanced.
[0105] The first SG-DBR region 535 is in an outside of the phase
adjusting region 533; while, the second SG-DBR region 537 is in an
outside of the gain region 531. Two SG-DBR regions, 535 and 537,
each has a length of bout 600 pm along the optical axis
thereof.
[0106] The first SG-DBR region 535 includes a portion of the
waveguide 535a, the upper cladding layer 512, the contact layer
535c and the anode electrode 535d, where each of layers is stacked
on the substrate 511 in this order; while, the second SG-DBR region
537 includes the waveguide 537a, the upper cladding Lauer 512, the
contact layer 537c, and the anode electrode 537d, where each of
layers is also stacked on the substrate 511 in this order.
[0107] Portions of the upper cladding layer 512 not converted with
the contact layer, 531c to 537c, and the anode electrodes, 531d to
537d, are covered with an insulating film made of, for instance,
silicon die-oxide (SiO.sub.2), which securely isolate respective
contact layers, 531c to 537c, and respective anode electrodes, 531d
to 537d.
[0108] The contact layers, 355c and 357c, and the anode electrodes,
355d and 357d, of the first and second SG-DBR regions, 355 and 357,
respectively, may be made of material same with those of the gain
region 531.
[0109] Furthermore, the first and second SG-DBR regions, 535 and
537, includes a sampled grating (SG) in an interface between the
waveguide, 535a or 537a, and the upper cladding layer 512, which
comprises of a plurality of grating regions and a plurality of
space regions alternately arranged to the others along the optical
axis. The SG shows a periodic reflectance spectrum.
[0110] Two SG-DBR regions, 535 and 537, may tune the emission
wavelength of the tunable LD 3A. Specifically, because two SG-DBR
regions, 535 and 537, forms the optical cavity for the photons
generated in the gain region 531, and the periodic reflectance
spectrum of the SG-DBR region, 535 and 537, depends on the injected
carriers; the emission wavelength of the tunable LD 3A may be tuned
by adjusting the bias conditions, namely, injected carriers of two
SG-DBR regions, 535 and 537. Ordinarily, the period of the periodic
reflectance spectrum of two SG-DBR regions, 535 and 537, are set to
be slightly different from the others. The laser oscillation may
occur at the wavelength where respective reflectance peaks of two
SG-DBR regions, 535 and 537, coincide with others, or the emission
wavelength may be tuned by adjusting the injected carriers so as to
emission wavelength becomes equal to the target wavelength.
[0111] FIG. 13 shows a block diagram to control the emission
wavelength of the wavelength lockable LD 1E of the present
embodiment, which is modified from those shown in FIG. 3. Although
previous embodiments concentrate of the tunable LD whose emission
wavelength may be tuned by varying the temperature thereof. While,
the LD 3A of the present embodiment may tune the emission
wavelength by adjusting the bias conditions; accordingly, the MCU
first sets the temperature of the tunable LD 3A by setting the bias
V2 to the TEC in a preset value and maintains the temperature
during the operation.
[0112] The MCU 121 then sets the biases, V31 to V37, in preset
conditions in order to set the emission wavelength roughly coincide
with one of the grid wavelength, which is called as the target
wavelength. The bias conditions, V31 to V37, are measured in
advance to the practical operation of the wavelength lockable LD 1E
and stored in a memory as a look-up-table. At this step, the
emission wavelength becomes nearly coincident with the target
wavelength but not exactly coincident therewith. The MCU then
precisely tunes the emission wavelength by monitoring the output of
the first PD.sub.1 and comparing the transmittance of the first
ring filter calculated from the monitored output of the first
PD.sub.1 with the designed transmittance. The MCU tunes the biases,
V33 to V37, such that the transmittance of the ring filter 51F
measured from the output of the first PD.sub.1 becomes coincident
with the designed value.
[0113] The MCU 121 then verifies the emission wavelength based on
the output of the second PD.sub.2, that is, when the transmittance
of the second ring filter 52F also has the periodic spectrum,
however, the period thereof is different from the others. Then,
transmittances of the second ring filter at the grid wavelengths
show a monotonic behavior; accordingly, the MCU 121 may verify
whether the emission wavelength is in the target wavelength or not,
even the transmittance of the first ring filter obtained from the
output of the first PD.sub.1 is in accordance with the designed
transmittance.
[0114] When the emission wavelength is tuned in a wavelength
different from the target wavelength, the MCU adjusts the biases,
V33 to V37, so as to make the transmittance of the second ring
filter 52F coincident with the designed transmittance. Thus, the
emission wavelength of the tunable LD 3A may be precisely tuned in
the target wavelength. After tuning the emission wavelength, the
MCU 121 sets other biases, V7 and V9, in respective conditions. The
bias V7 applied to the SOA 7 may adjust the magnitude of the output
from the wavelength lockable LD 1E, while, the condition V9 to the
optical modulator 9 may drive ideally.
Sixth Embodiment
[0115] The present invention does not restrict the tunable LD in an
arrangement shown in FIG. 11. FIG. 14 is a cross section of still
another tunable LD 3B, which includes a modified gain region 631
with a structure of the sampled grating distributed feedback
(SG-DFB) structure, and a chirped SG-DBR regions 635. Specifically,
The SG-DFB region 631 includes, on the semiconductor substrate 611,
the lower cladding layer 631e, the active layer 631a, the upper
cladding layer 631b, the contact layer 631c and the anode electrode
631d. The SG-DFB region 631 further includes a plurality of grating
regions 631A and a plurality of space regions 631B alternately
arranged to each other along the optical axis. One grating region
631A and one space region 631B continuous to the grating region
631A constitutes one segment; and the SG-DGB region includes a
plurality of segments. The optical grating in the grating regions
631A may be made of material different from that of the lower
cladding layer 631e and buried within the lower cladding layer
631e. When the lower cladding layer is made of InP, the grating
optical grating may be made of, for instance,
Ga.sub.0.22In.sub.0.78As.sub.0.47P.sub.0.53.
[0116] On the other hand, the CSG-DBR region 635 includes, on the
semiconductor substrate 611, the lower cladding layer 631e, the
waveguide layer 635a, the upper cladding layer 631b, the insulating
film 635c, and a plurality of heaters, 635Ah to 635Ch. The
waveguide layer 635a may be made of
Ga.sub.0.22In.sub.0.78As.sub.0.4P.sub.0.53, which has a shorter
bandgap wavelength compared with that of the active layer 631a in
the gain region 631. That is, the waveguide layer 635a in the
CSG-DBR region 635 is substantially transparent to the light
generated in the gain region 631. Each of heaters, 635Ah to 635Ch,
has electrodes, 635Ae to 635Ce. The lower cladding layer 631e and
the upper cladding layer 631b of the gain region 631 extend in the
CSG-DBR region 635, namely, these two layers, 631e and 631b, are
common in the gain region 631 and the CSG-DBR region 635. The
CSG-DBR region 635 includes three blocks, 635A to 635C. Each of
blocks has a plurality of segments comprised of a grating region
631A and the space region 631B as those in the gain region 631. A
feature of the CSG-DBR region 635 is that at least one block, 635A
to 635C, has an optical length of the space region different from
the optical length of the other blocks, and each of blocks, 635A to
635C, accompanies with a monolithic heater, 635Ah to 635Ch, in the
top of the device.
[0117] While, as described above; each segment in the gain region
631 has the same optical length. Accordingly, the SG-DFB region 631
may have a plurality of gain peaks with a constant pitch, while,
the CSG-DBR region 635 includes three units, 635A to 635c, at least
one of the units has a specific optical length in the space region
631b, and the optical length may be varied by the temperature
controllable by respective heaters, 635Ah to 635Ch. Therefore, the
laser oscillation may occur in a strict condition where one of the
discrete gain peaks in the SG-DFB region 631 coincides with one of
the reflectance peaks in the CSG-DBR region. Moreover, because the
reflectance peaks in the CSG-DBR region 635 may be controlled by
adjusting the temperature of respective blocks, 635A to 635C, one
of segments dominates the wavelength tuning, which may not only
tune the emission wavelength precisely but widen the tuning range
of the emission wavelength.
[0118] The tunable LD 3B of the present embodiment may further
include an absorption region 639 in a side opposite to the gain
region 631 with respect to the CSG-DBR region 635. That is, the
absorption region 639 and the gain region 631 put the CSG-DBR
region 635 therebetween. The absorption region 639 includes, on the
substrate 611, the lower cladding layer 631e, the absorption layer
639a, the upper cladding layer 631c, the contact layer 639c and the
electrode 639d. The absorption layer 639a may be made of material
that can absorb the light emitted in the gain region 631. The
absorption layer 639a preferably has a bandgap wavelength longer
than the emission wavelength of the LD 3B. Further preferably, the
bandgap wavelength of the absorption layer is longer than a longest
wavelength at which the tunable LD 3B may oscillate. Specifically,
the absorption layer 639a may have the MQW structure constituted by
well layers made of Ga.sub.0.47In.sub.0.53As with a thickness of 5
nm and barrier layers made of
Ga.sub.0.28In.sub.0.72As.sub.0.61P.sub.0.39 with a thickness of 10
nm. The absorption layer 639a may be made of a bulk of
.sub.Ga0.46In.sub.0.54AS.sub.0.98P.sub.0.02 The embodiment shown in
FIG. 3B has the absorption layer 639a made of the same material
with those of the active layer 631a of the gain region 631. The
absorption layer may have the function to absorb light generated in
the gain region 631 but leaked through the CSG-DBR region 635. When
the leaked light increases, the light reflected at the face of the
absorption region 639 backs in the gain region 641, which becomes
an optical noise and degrades the emission characteristic of the
tunable LD 3B. Varying the bias condition applied to the electrode
639d, the absorption characteristic of the region 639 may be widely
changed. When the tunable LD 3B integrates the absorption region
639, the wavelength monitor according to the present invention may
optically couple with the front side of the gain region 631.
[0119] While particular embodiments of the present invention have
been described herein for purposes of illustration, many
modifications and changes will become apparent to those skilled in
the art. Accordingly, the appended claims are intended to encompass
all such modifications and changes as fall within the true spirit
and scope of this invention.
* * * * *