Wavelength Tunable Optical Transmitter

Abstract

In a DBR laser of a wavelength-tunable transmitter, a rear DBR region, an active region, and a front DBR region are integrated along an optical axis direction. The diffraction grating structure is set so that an oscillation mode using a reflection peak on the shortest wavelength side among a plurality of reflection peaks corresponding to the wavelength-tunable band is easily oscillated the most in a state where a current to the two DBR regions of the SSG-BPFR is 0. The SSG-DBR laser is configured such that the average period value of the diffraction grating of the front DBR is larger than the average period value of the diffraction grating of the rear DBR. The diffraction grating is configured so that the wavelengths of the reflection peaks on the shortest wavelength side among the plurality of reflection peaks coincide with each other between the two DBR regions in a state where no current is supplied.

Claims

1. A wavelength-tunable optical transmitter in which a wavelength-tunable light source and a field-absorption optical modulator that is optically connected to a forward DBR region are integrated along an optical axis direction, the wavelength-tunable light source including a rear DBR region with a first diffraction grating and a reflection characteristic consisting of a plurality of reflection peaks, an active region producing an optical gain, and the front DBR region with a second diffraction grating and a reflection characteristic consisting of a plurality of reflection peaks, wherein a wavelength interval of the reflection peaks of the front DBR region set to be greater than a wavelength interval of the reflection peaks in the rear DBR region, and an average period A.sub.0_front of the first diffraction grating is set to be greater than an average period .sub.0_rear of the second diffraction grating.

2. The wavelength-tunable optical transmitter according to claim 1, wherein the first diffraction grating and the second diffraction grating are configured such that a wavelength of a reflection peak of a shortest wavelength among the plurality of reflection peaks of the rear DBR region coincides with a wavelength of a reflection peak of a shortest wavelength among the plurality of reflection peaks of the front DBR region when a first injection current to the rear DBR region and a second injection current to the front DBR region are zero.

3. The wavelength-tunable optical transmitter according to claim 1, wherein an oscillation wavelength of the wavelength-tunable light source is changed by injecting a first injection current into the front DBR region and a second current into the rear DBR region.

4. The wavelength-tunable optical transmitter according to claim 1, wherein a variable range of an oscillation wavelength includes a C band or an L band, the front DBR region and the rear DBR region have the same number of reflection peaks N, and the .sub.first diffraction grating and the second diffraction grating are configured so as to satisfy the following equations for an average period difference .sub.0 between an average period .sub.0_front of the first diffraction grating and an average period .sub.0_rear of the second diffraction grating. ${}_0=\frac{{}_{0\_\mathrm{front}}-{}_{0\_\mathrm{rear}}}{{}_{0\_\mathrm{front}}}100\mathrm{and}0.04<\frac{2{}_0}{N-1}<0.09\left(\%\right)$

5. The wavelength-tunable optical transmitter according to claim 1, wherein a variable range of an oscillation wavelength includes an O band, the .sub.front DBR region and the rear DBR region have the same number of reflection peaks N, and the .sub.fir'st diffraction grating and the second diffraction grating are configured so as to satisfy the following equations for an average period difference .sub.0 between an average period .sub.0 front of the first diffraction grating and an average period .sub.0_rear of the second diffraction grating. ${}_0=\frac{{}_{0\_\mathrm{front}}-{}_{0\_\mathrm{rear}}}{{}_{0\_\mathrm{front}}}100\mathrm{and}0.03<\frac{2{}_0}{N-1}<0.06\left(\%\right)$

6. The wavelength-tunable optical transmitter according to claim 1, wherein a semiconductor optical amplifier (SOA) is further integrated on an output side of the field-absorption optical modulator.

7. The wavelength-tunable optical transmitter according to claim 1, wherein a wavelength range of the plurality of reflection peaks of the first diffraction grating and a wavelength range of the plurality of reflection peaks of the second diffraction grating are included at least in a wavelength-tunable range by the wavelength-tunable light source.

8. The wavelength-tunable optical transmitter according to claim 2, wherein a semiconductor optical amplifier (SOA) is further integrated on an output side of the field-absorption optical modulator.

9. The wavelength-tunable optical transmitter according to claim 3, wherein a semiconductor optical amplifier (SOA) is further integrated on an output side of the field-absorption optical modulator.

10. The wavelength-tunable optical transmitter according to claim 4, wherein a semiconductor optical amplifier (SOA) is further integrated on an output side of the field-absorption optical modulator.

11. The wavelength-tunable optical transmitter according to claim 5, wherein a semiconductor optical amplifier (SOA) is further integrated on an output side of the field-absorption optical modulator.

12. The wavelength-tunable optical transmitter according to claim 2, wherein a wavelength range of the plurality of reflection peaks of the first diffraction grating and a wavelength range of the plurality of reflection peaks of the second diffraction grating are included at least in a wavelength-tunable range by the wavelength-tunable light source.

13. The wavelength-tunable optical transmitter according to claim 3, wherein a wavelength range of the plurality of reflection peaks of the first diffraction grating and a wavelength range of the plurality of reflection peaks of the second diffraction grating are included at least in a wavelength-tunable range by the wavelength-tunable light source.

14. The wavelength-tunable optical transmitter according to claim 4, wherein a wavelength range of the plurality of reflection peaks of the first diffraction grating and a wavelength range of the plurality of reflection peaks of the second diffraction grating are included at least in a wavelength-tunable range by the wavelength-tunable light source.

15. The wavelength-tunable optical transmitter according to claim 5, wherein a wavelength range of the plurality of reflection peaks of the first diffraction grating and a wavelength range of the plurality of reflection peaks of the second diffraction grating are included at least in a wavelength-tunable range by the wavelength-tunable light source.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0043] FIG. 1 is a diagram showing a schematic configuration of a general EADFB laser.

[0044] FIG. 2 is a diagram schematically showing an extinction curve and an intensity modulation principle of the EADFB laser.

[0045] FIG. 3 is a diagram showing a configuration of an AXEL with SOAs integrated in the EADFB laser.

[0046] FIG. 4 is a diagram showing a schematic cross-sectional structure of a general DBR laser.

[0047] FIG. 5 is a diagram for explaining control of a reflection spectrum and an oscillation wavelength of a DBR region.

[0048] FIG. 6 is a schematic diagram for explaining a diffraction grating structure of an SSG-DBR.

[0049] FIG. 7 is a diagram for explaining a behavior of an injected current-reflection peak of an SSG-DBR laser.

[0050] FIG. 8 is a conceptual diagram for explaining a modulation operation principle of an EA modulator.

[0051] FIG. 9 is a diagram for explaining optical output fluctuations of a wavelength-tunable AXEL associated with carrier injection.

[0052] FIG. 10 is an explanatory diagram of an SSG-DBR laser operation of the wavelength-tunable optical transmitter of the present disclosure.

[0053] FIG. 11 is a diagram showing a cross-sectional configuration of a wavelength-tunable optical transmitter of Example 1.

[0054] FIG. 12 is a diagram showing an optical output intensity of a sample B by a diffraction grating of the prior art.

[0055] FIG. 13 is a diagram showing an optical output intensity of a sample A by a diffraction grating of Example 1.

[0056] FIG. 14 is a diagram showing an optical output intensity of a wavelength-tunable optical transmitter of Example 2.

DESCRIPTION OF EMBODIMENTS

[0057] A wavelength-tunable optical transmitter of the present disclosure includes at least a transmission function of integrating a DBR laser and an EA modulator, modulating light generated by the DBR laser by an information signal in the EA modulator, and transmitting the modulated optical signal. In the DBR laser of the wavelength-tunable transmitter of the present disclosure, a rear DBR region, an active region, and a front DBR region are integrated on a semiconductor substrate in this order along an optical axis direction. The DBR laser is an SSG-DBR having a plurality of reflection peaks in both the rear DBR region and the front DBR region. A diffraction grating structure is set so that an oscillation mode using a reflection peak on the shortest wavelength side among a plurality of reflection peaks corresponding to the wavelength-tunable band is easily oscillated the most in a state where a current to the two DBR regions of the SSG-DBR is 0.

[0058] In the wavelength-tunable transmitter of the present disclosure, a DBR laser is configured such that an average period value of a diffraction grating of the front DBR is larger than an average period value of a diffraction grating of the rear DBR. The diffraction grating is configured so that the wavelengths of the reflection peaks on the shortest wavelength side among the plurality of reflection peaks coincide with each other between the two DBR regions in a state where no current is supplied to the two front and rear DBR regions. In the SSG-DBR laser of the prior art, the average period value of the diffraction grating in the rear DBR region and the average period value of the diffraction grating in the front DBR region are designed to be the same. Furthermore, in a state in which no current flows through the two DBR regions, the wavelength of the central reflection peak among the plurality of reflection peaks in the rear DBR region coincides with the wavelength of the central reflection peak among the plurality of reflection peaks in the front DBR region. The configuration and operation of the SSG-DBR laser in the wavelength-tunable optical transmitter of the present disclosure will be described below with reference to the configuration of the diffraction grating of the SSG-DBR laser of the prior art.

[0059] FIG. 10 is a diagram for explaining an SSG-DBR laser operation of the wavelength-tunable optical transmitter of the present disclosure. The SSG-DBR laser of the wavelength-tunable optical transmitter of the present disclosure is configured to oscillate at a reflection peak wavelength on the short wavelength side among a plurality of reflection peaks in a state where no DBR current flows. In this state, the optical loss due to the free carriers is minimized. The upper diagram of FIG. 10 shows reflection characteristics of two DBR regions in a state where no DBR current flows. The lower diagram of FIG. 10 shows the wavelength dependency of the optical output of the SSG-DBR laser of the wavelength-tunable optical transmitter of the present disclosure. The two diagrams of FIG. 10 correspond to the two diagrams of FIG. 9(b) in terms of the SSG-DBR laser of the prior art, and the description will be made with comparison with the configuration of the prior art.

[0060] First, the wavelength dependency of the optical output of the SSG-DBR laser will be outlined with reference to the lower diagram of FIG. 10. In the optical output of the SSG-DBR laser of the wavelength-tunable optical transmitter of the present disclosure, fine light intensity fluctuations of saw teeth-like repetition corresponding to the number of reflection peaks in the DBR regions can be seen. However, the overall wavelength-tunable bandwidth has a right-downward sloping characteristic, and the optical output tends to be higher at shorter wavelengths and gradually decreases toward the longer wavelengths. Among the repeated optical output fluctuations, the left-downward optical output fluctuation within one saw tooth corresponds to one of the plurality of reflection peaks in each of the two DBR regions. Thus, as in the case of the prior art in FIG. 9(b), oscillations occur using the same reflection peak within the wavelength range corresponding to one saw tooth.

[0061] The wavelength dependency of the optical output of the SSG-DBR laser of the wavelength-tunable optical transmitter of the present disclosure represents a change in the optical output dependent on the current in the front DBR region and the current in the rear DBR region. The optical output characteristic of a right-downward shift across the wavelength-tunable band in FIG. 10 is achieved by setting the structure of each diffraction grating to oscillate in the shorter wavelength side oscillation mode with as little as possible of the two DBR currents, as described below. By setting the diffraction grating structure specific to the present invention, the SSG-DBR laser of the wavelength-tunable optical transmitter of the present disclosure can obtain a higher optical output on the short wavelength side than on the long wavelength side. On the other hand, the EA modulator tends to have greater optical loss on the shorter wavelength side, as described in FIG. 8, and has an overall left-downward sloping characteristic. The right-downward optical output characteristic of the SSG-DBR laser of the present disclosure and the left-downward optical output characteristic of the EA modulator are offset, and the optical output of the wavelength-tunable optical transmitter of the present disclosure provides a generally flat optical output characteristic over the entire wavelength range. Next, a more specific design of the diffraction grating in the SSG-DBR laser of the present disclosure will be described.

[0062] In determining the reflection characteristics of the SSG-DBR, the number of reflection peaks and the wavelength interval of the reflection peaks can be designed arbitrarily (NPL 5). In the SSG-DBR laser, the wider the wavelength interval of the reflection peaks or the larger the number of the reflection peaks is, the wider the wavelength range can be controlled. However, when the wavelength interval of the reflection peaks is widened, it is difficult to control the oscillation wavelength to the wavelength between the reflection peaks, and a wavelength gap which cannot be controlled in wavelength occurs between the reflection peaks. Further, if the number of reflection peaks is increased, the reflective index of one reflection peak decreases, and it becomes difficult to maintain laser oscillation. In order to widen the wavelength-tunable range, there are limits to increase in the number of reflection peaks and increase in the wavelength interval of reflection peaks in order to obtain pseudo continuous wavelength-tunable characteristics and stable laser oscillation.

[0063] Considering the characteristics of the SSG-DRB laser, it is necessary to set the number N of realistic reflection peaks to 5 to 11 in the wavelength-tunable band assumed in each band. The optimum value of the wavelength interval of the reflection peak varies depending on the oscillation wavelength band, and at a C band wavelength band (1530 to 1565 nm) or an L band wavelength band (1565 to 1625 nm), the wavelength interval of adjacent reflection peaks in the two DBR regions (wavelength difference) is set to 4 to 9 nm, respectively.

[0064] On the other hand, in an 0 band wavelength band (1260 to 1360 nm), in principle the refractive index change amount is smaller and the Bragg wavelength shift amount is smaller as compared with the C band. For this reason, it is necessary to set the wavelength interval between adjacent reflection peaks in the two DBR regions to 3 to 6 nm. When matching the peak wavelength of the shortest wavelength among the plurality of reflection peaks that the two DBR regions each have, the number of reflection peaks and the wavelength interval condition of the reflection peaks in the SSG-DBR described above must be considered.

[0065] Referring again to FIG. 10, a specific configuration of the diffraction grating of the SSG-DBR laser in the wavelength-tunable optical transmitter of the present disclosure will be described. FIG. 10 shows an example of reflection peak setting in the SSG-DBR laser of the present disclosure. As an example of the general structure of the C-band wavelength band, the number of reflection peaks N is 7 in the 1.55 m wavelength band, the reflection peak interval in the front DBR region is .sub.front=8.6 nm, and the reflection peak interval in the rear DBR region is .sub.rear=7.7 nm. In FIG. 10, the reflection characteristics of the front DBR region and the reflection characteristics of the rear DBR region are shown by solid lines and broken lines respectively, in a state where no current flows through the two DBR regions. What is characteristic of the SSG-DBR laser of the present disclosure is that the wavelength at which the reflection peaks coincide between the two DBR regions with no current flowing in the DBR regions is the reflection peak on the shortest wavelength side of the plurality of reflection peaks. In other words, the diffraction grating is set so that the reflection peak wavelength 91 on the shortest wavelength side among the plurality of reflection peaks in the front DBR region coincides with the reflection peak wavelength 91 on the shortest wavelength side among the plurality of reflection peaks in the rear DBR region.

[0066] It is necessary that the wavelength range including the plurality of reflection peaks in the two DBR regions includes at least the desired wavelength-tunable range in the wavelength-tunable transmitter in a state where no current flows in the DBR regions. This is because the reflection peak on the longest wavelength side of the plurality of reflection peaks shifts only to the short wavelength side even if a current is made to flow in the DBR regions, and therefore the oscillation wavelength cannot be adjusted to the longer wavelength side than the reflection peak on the longest wavelength side.

[0067] Specifically, the average value of the diffraction grating period is adjusted so that the wavelengths of reflection peaks on the shortest wavelength side coincide with each other with respect to the two DBR regions. In the configuration example shown in FIG. 10, the diffraction grating structure is set so that the average value .sub.0_front of the diffraction grating periods of the front DBR region is 0.23% larger than the average value .sub.0_rear of the diffraction grating periods of the rear DBR region. Thus, the entire reflection characteristics of the front DBR region are gradually shifted to the longer wavelength side in relation to the rear DBR region, and a wider reflection peak arrangement is obtained. The period of the diffraction grating represents the physical length (pitch) of the repetition of the repeating structure of the formed bumps and dips on the top surface of the active layer, which has a dimension of length. Note that the normal term period differs from the one having the dimension of time.

[0068] Here, the reflection characteristics of the SSG-VDBR laser of the prior art shown in FIG. 9 and the reflection characteristics of the SSG-VDBR laser in the wavelength-tunable optical transmitter of the present disclosure shown in FIG. 10 will be compared with each other. In the SSG-BLD DBR laser of the prior art, the average values of the diffraction grating periods of the two DBR regions are designed to be the same. For this reason, as shown in FIG. 9(b), the wavelengths of the reflection peaks located at the center among the plurality of reflection peaks corresponding to the wavelength-tunable band coincide with each other. When a wavelength 92 of a reflection peak on the shortest wavelength side of the rear DBR region is compared with a wavelength 93 of a reflection peak on the shortest wavelength side of the front DBR region in a state where no current flows to the two DBR regions, a wavelength difference is approximately 3.6 nm.

[0069] On the other hand, in the SSG-DBR laser in the wavelength-tunable optical transmitter of the present disclosure shown in FIG. 10, the wavelength of the reflection peak on the shortest wavelength side of the rear DBR region and the wavelength of the reflection peak on the shortest wavelength side of the front DBR region are made to coincide with each other. In this manner, in the two DBR regions, if the wavelengths of the reflection peaks on the shortest wavelength side among the plurality of reflection peaks coincide with each other, the mode on the shortest wavelength side oscillates in a state where no DBR current flows. Also, when the oscillation wavelength is finely adjusted by using the mode on the shortest wavelength side, the wavelength can be adjusted by a relatively smaller DBR current than the prior art. Therefore, in the oscillation mode on the shortest wavelength side, the reduction in the optical output due to the application of the DBR current for wavelength adjustment is greatly suppressed. This is in contrast to the configuration of the prior art illustrated in FIG. 9(b), which required the maximum DBR current when adjusting the wavelength toward the shortest wavelength.

[0070] Therefore, the wavelength-tunable optical transmitter of the present invention is a wavelength-tunable optical transmitter in which a wavelength-tunable light source and a field-absorption optical modulator that is optically connected to a forward DBR region are integrated along an optical axis direction, the wavelength-tunable light source including a rear DBR region with a first diffraction grating and a reflection characteristic consisting of a plurality of reflection peaks, an active region producing an optical gain, and the front DBR region with a second diffraction grating and a reflection characteristic consisting of a plurality of reflection peaks, wherein a wavelength interval of the reflection peaks of the front DBR region set to be greater than a wavelength interval of the reflection peaks in the rear DBR region, and an average period .sub.0_front of the first diffraction grating is set to be greater than an average period .sub.0_rear of the second diffraction grating.

[0071] The first diffraction grating and the second diffraction grating can be configured such that a wavelength of a reflection peak of a shortest wavelength among the plurality of reflection peaks of the rear DBR region coincides with a wavelength of a reflection peak of a shortest wavelength among the plurality of reflection peaks of the front DBR region when a first injection current to the rear DBR region and a second injection current to the front DBR region are zero.

[0072] In the SSG-DBR laser of the present disclosure, preferred configurations of the two diffraction grating structures for each target wavelength-tunable band are as follows. Suppose that the average periods of the diffraction gratings in the front and rear DBR regions are .sub.0_front and .sub.0_rear respectively. Using the two average periods, the average period difference .sub.0 between the two gratings is defined as in the following equation.

[00001]$\begin{array}{cc}{}_0=\frac{{}_{0\_\mathrm{front}}-{}_{0\_\mathrm{rear}}}{{}_{0\_\mathrm{front}}}100& \mathrm{Equation}\left(3\right)\end{array}$

[0073] Assuming that the number of reflection peaks corresponding to the target wavelength-tunable range is N, when the oscillation wavelength is in the 1.55 m band (C band wavelength band, L band wavelength band), the diffraction grating periods of the front DBR region and the rear DBR region are preferably designed to satisfy the following equation.

[00002]$\begin{array}{cc}\frac{2{}_0}{N-1}=0.040.09\%& \mathrm{Equation}\left(4\right)\end{array}$

[0074] Furthermore, when the oscillation wavelength is in the 1.3 m band (O-band wavelength band), the change in refractive index due to the carrier-plasma effect is smaller than in the 1.5 m band. Therefore, it is preferable to design the diffraction grating periods of the front DBR region and the rear DBR region so as to satisfy the following equation.

[00003]$\begin{array}{cc}\frac{2{}_0}{N-1}=0.030.06\%& \mathrm{Equation}\left(5\right)\end{array}$

[0075] A specific example of the wavelength-tunable optical transmitter including the SSG-DBR laser will be described below with reference to a specific configuration and improvement in wavelength dependency of the optical output level.

Example 1

[0076] FIG. 11 is a diagram showing a cross-sectional configuration of a wavelength-tunable optical transmitter of Example 1. A wavelength-tunable optical transmitter 500 is a wavelength-tunable AXEL in which SOA is integrated in addition to an SSG-DBR laser and an EA modulator. In the SSG-DBR laser, an active region 120 having a length of 300 m, a front DBR region 100b having a length of 200 m, and a rear DBR region 100a having a length of 400 m are configured in an optical axis direction. Further, an EA modulator 130 having a length of 200 m and an SOA 140 having a length of 150 m are integrated in front of the SSG-DBR laser along the optical axis direction, and the entire wavelength-tunable optical transmitter is configured as a monolithic integrated element. A phase adjustment region 110 is also provided between the active region 120 and the rear DBR region 100a. A modulated optical signal 4 is output from a substrate end surface on the SOA 140 side.

[0077] The manufacturing process of the wavelength-tunable optical transmitter 500 will now be described. An initial substrate in which a lower SCH (Separated Confinement Heterostructure) layer, an active layer of a multiple quantum well layer (MQW 1), and an upper SCH layer are sequentially grown on an n-InP substrate, is used for manufacturing the element. The multiple quantum well layer has an optical gain in an oscillation wavelength of 1.55 m band. First, leaving a part that becomes the active region of the DBR laser and the SOA region, other active layers are selectively etched, and a multiple quantum well layer (MQW 2) for the EA modulator is grown by butt joint regrowth. Subsequently, leaving the active region of the DBR laser, the EA modulator region, and the SOA region, selective etching and butt joint regrowth are performed again to form a core layer of the passive waveguide. Next, an SSG-DBR diffraction grating that operates in the oscillation wavelength band of 1.55 m and has an average period that satisfies the above equations (3) and (4) was formed in the two DBR regions. Thereafter, a p-InP clad layer was grown over the entire surface of the element by re-growth. In this example, the thickness of the cladding layer is set to 2.0 m, and the electrode region is designed so that the light field is not applied.

[0078] After the cladding layer was grown, the mesa structure was formed by etching to form a ridge waveguide structure. Thereafter, a p-side electrode was formed on the upper surface of the semiconductor substrate. Thereafter, the InP substrate is polished to approximately 150 m, and an electrode is formed on the rear surface of the substrate, completing the process on the semiconductor wafer. In the wavelength-tunable optical transmitter of the present example, the two DBR regions and the passive waveguide region have the same core layer formed by butt-joint growth, and the only difference in layer structure between these regions is the presence or absence of the diffraction grating. The active region and the SOA region also have a multi-quantum well layer of the same structure and are grown collectively. Thus, despite the structure in which a plurality of regions are integrated, the number of regrowth cycles can be reduced, and low-cost manufacturing is made possible.

[0079] Here, the structure of the diffraction gratings (SSG) formed in the two DBR regions 100a and 100b of the wavelength-tunable optical transmitter of Example 1 will be described. As described in FIG. 10, in the SSG-DBR laser of Example 1, the two DBR regions 100a and 100b have a plurality of reflection peaks, respectively, and the reflection peak intervals are slightly different between the two DBR regions. By the vernier effect, one peak among the plurality of reflection peaks can be selected in each of the two DBR regions to control the oscillation wavelength.

[0080] The two DBR regions 100a and 100b each have seven reflection peaks (N=7). The reflection peak intervals in the front DBR region and the rear DBR region are set to be .sub.front=8.6 nm and .sub.rear=7.7 nm, respectively, and the reflection peak intervals in the front DBR region are designed to be slightly larger than the reflection peak intervals in the rear DBR region. In addition, the average period .sub.0_front in the diffraction grating in the front DBR region is designed to be slightly greater than the average period .sub.0_rear in the diffraction grating in the rear DBR region. From the values of the reflection peak intervals .sup.front and .sup.rear in the front DBR region and the rear DBR region and the number of reflection peaks N=7, the average periods .sub.0_front and .sub.0_rear of the respective diffraction gratings in the front DBR region and the rear DBR region are determined to satisfy the equation (4). As shown in the equation (4) for the 1.55 m band, .sub.0_front is designed to be 0.174% greater than .sub.0_rear (.sub.0=0.174).

[0081] By setting the diffraction grating structure as described above, the wavelengths of the reflection peaks of the shortest wavelength out of the plurality of reflection peaks of the two DBR regions coincide with each other and are brought into a resonant state in a state where no DBR current is injected into any of the two DBR regions.

[0082] A wavelength-tunable AXEL in which SSG-DBR lasers having the above-mentioned specific diffraction gratings are integrated was manufactured and evaluated. The element of the structure of the present example is taken as a sample A. In the present example, in order to confirm the improvement effect of the optical output characteristics by the SSG-DBR lasers having a specific diffraction grating, a wavelength-tunable AXEL having the same diffraction grating structure as that of the prior art was manufactured. In other words, devices having the same average value of diffraction grating periods in two DBRs were also manufactured and the same evaluation was performed. The element according to the configuration of the prior art is taken as a sample B. The samples A and B have the same structure except for the diffraction grating structure, and were fabricated using the same fabrication process.

[0083] The modulation characteristics of each oscillation wavelength were evaluated in each of the fabricated devices. The entire wavelength-tunable range was divided into channels at 100 GHz intervals, and the modulation characteristics were evaluated when the device was controlled to the corresponding wavelength of each channel. As for the SSG-DBR laser of each element, a current of 90 mA was injected into the active region and the SOA region, and the front DBR region, the rear DBR region, and the phase adjustment region were controlled, thereby adjusting the wavelength of each channel. The adjustment of the driving conditions in each channel was carried out under the condition that the oscillation wavelength was adjusted with accuracy of +/0.01 nm with respect to the target wavelength, and that the optical output was maximized in a range satisfying SMSR >45 dB.

[0084] The EA modulator receives a modulation signal having a transmission rate of 10 Gbit/s, a signal format of NRZ, and a signal sequence of PRBS 2.sup.311, and has an amplitude voltage of 2.0 V at all times. For the DC bias voltage to the EA modulator, the EYE pattern waveform of the modulated optical signal was evaluated and adjusted to a value to maximize the dynamic extinction ratio. The absolute value of the voltage applied to the actual EA modulator tends to be smaller toward the short wavelength side channel and larger toward the long wavelength side. This tendency of the modulation signal is because the absorption curve of the EA modulator has a larger absorption toward the shorter wavelength side as described with reference to FIG. 8. The evaluated wavelength channels are 49 channels in both the sample A and the sample B.

[0085] The EYE pattern waveform was evaluated to obtain a relatively clear EYE aperture over all channels for both of the samples A and B, and a dynamic extinction ratio of 6 dB or more was confirmed over all channels. In order to confirm the effect of the SSG-DBR laser having the above-mentioned specific diffraction grating, the optical output intensities of all channels measured in the samples A and B were compared.

[0086] FIG. 12 is a diagram showing the optical output intensity of the sample B by the diffraction grating with the configuration of the prior art. Over the entire wavelength-tunable band, there are seven fluctuations in the optical output in the form of sawteeth. These fluctuations in the optical output indicate that the oscillation state is determined by one of the seven different reflection peaks in each of the two DBR regions. The point within one sawtooth range corresponds to the oscillation state set by different DBR currents using the same reflection peak. Within one sawtooth range, the characteristic of the leftward drop in which the optical output decreases as the DBR current increases. It is possible to confirm the tendency of the leftward decrease in which the optical output decreases toward the shorter wavelength side over the entire wavelength-tunable band. The leftward decrease tendency is derived from the wavelength dependency of the optical loss of the EA modulator as described with reference to FIG. 8. The leftward decrease of the optical output is a problem to be solved. A maximum optical output of 8.1 dBm was obtained in a channel approximately in the center of the wavelength-tunable range. On the other hand, the minimum optical output was obtained by the channel of the shortest wavelength, and the optical output thereof was approximately 4 dBm. Therefore, the optical output between channels has a fluctuation with a maximum width of 12.1 dB in the entire wavelength-tunable range. In this manner, when the wavelength-tunable AXEL is constituted by the SSG-DBR laser using the diffraction grating having the configuration of the prior art, a very large fluctuation in the optical output is caused depending on the wavelength.

[0087] FIG. 13 is a diagram showing the optical output intensity of the sample A by the diffraction grating having the configuration of the present disclosure. In the sample A as well, it is possible to confirm seven fine optical output fluctuations representing optical fluctuations in the seven SSG modes. However, unlike the sample B of the prior art shown in FIG. 12, there is no significant decrease in the optical output on the short wavelength side, and a uniform optical output is obtained over the entire wavelength-tunable range. The maximum value of the optical output was 4.2 dB, which was slightly lower than that of the sample B according to the prior art, but the total optical output fluctuation width was 5.3 dB at maximum, which was reduced from 12.1 dB of the sample B according to the prior art by 7 dB. This is caused by an improvement in the reduction of the optical output on the short wavelength side. The optical loss in the EA modulator has the same tendency for both of the samples A and B because of the same configuration. The wavelength dependency of the optical loss in the EA modulator was compensated by designing the diffraction grating so that the optical output from the laser was maximized on the short wavelength side by adopting the SSG-DBR structure of the present disclosure. A uniform optical output was obtained in the entire wavelength-tunable optical transmitter.

Example 2

[0088] The present example describes a wavelength-tunable optical transmitter in which the oscillation wavelength is set to 1.3 m band and which corresponds to high-speed modulation of 25 Gbit/s class. Since the basic structure of the device of the present embodiment is the same as that of the device of Example 1 shown in FIG. 11, description thereof is omitted accordingly.

[0089] Compared with the operation in the 1.55 m band of Example 1, the operation in the 1.3 m band of the present example requires designing the reflection peak intervals of the two DBR regions to be smaller. This is because in the 1.3 m band, the amount of change in refractive index and the amount of wavelength shift due to the carrier plasma effect are smaller than those in the 1.55 m band. In the present example, the two DBR regions each have nine reflection peaks (N=9). The reflection peak intervals in the front DBR region and the rear DBR region are .sub.front=4.0 nm and .sub.rear=3.5 nm, respectively, and the reflection peak intervals of the front DBR region are slightly greater than the reflection peak intervals of the rear DBR region. In addition, the average period .sub.0_front in the diffraction grating in the front DBR region is designed to be slightly greater than the average period .sub.0_rear in the diffraction grating in the rear DBR region. From the values of the reflection peak intervals .sub.front and .sub.rear in the front DBR region and the rear DBR region and the number of reflection peaks N=9, the average periods .sub.0_front and .sub.0_rear of the respective diffraction gratings in the front DBR region and the rear DBR region are determined to satisfy the equation (5). .sub.0_front is designed to be 0.154% greater than .sub.0_rear (.sub.0=0.154).

[0090] By setting the diffraction gratings to satisfy the equation (5) as described above, the reflection peaks of the shortest wavelength among the plurality of reflection peaks of the two DBR regions coincide with each other in the state where no DBR current is injected, resulting in a state of resonance. In the same manner as in Example 1, a wavelength-tunable AXEL in which SSG-DBR lasers are integrated was prepared and evaluated.

[0091] In each device manufactured in the same manner as in Example 1, the modulation characteristics for each oscillation wavelength were evaluated. The entire wavelength-tunable range was divided into channels at 100 GHz intervals, and the modulation characteristics were evaluated when the device was controlled to the corresponding wavelength of each channel. As for the SSG-DBR laser of each element, a current of 90 mA was injected into the active region and a current of 120 mA was injected into the SOA region, and the front DBR region, the rear DBR region, and the phase adjustment region were controlled independently, thereby adjusting the wavelength of each channel. The adjustment of the driving conditions in each channel was carried out under the condition that the wavelength was adjusted with accuracy of +/0.01 nm with respect to the target wavelength, and that the optical output was maximized in a range satisfying SMSR >45 dB.

[0092] The EA modulator receives a modulation signal having a transmission rate of 25 Gbit/s, a signal format of NRZ, and a signal sequence of PRBS 2.sup.311, and has an amplitude voltage of 1.5 V at all times. For the DC bias voltage to the EA modulator, the EYE pattern waveform of the modulated optical signal was evaluated and adjusted to a voltage value to maximize the dynamic extinction ratio. The absolute value of the voltage applied to the actual EA modulator tends to be smaller toward the short wavelength side channel and larger toward the long wavelength side. As with Example 1, this is because the absorption curve of the EA modulator has a larger absorption toward the shorter wavelength side. The evaluated wavelength channels are 55 channels in both the sample A and the sample B. The Eye pattern waveform of each channel was evaluated to obtain a clear EYE aperture in all channels. It was confirmed that the dynamic extinction ratio was 5.5 dB or greater over the entire channels.

[0093] FIG. 14 is a diagram showing the optical output intensity of of a wavelength-tunable optical transmitter of Example 2. In the entire wavelength-tunable range, it is possible to confirm nine fine optical output fluctuations representing optical fluctuations in an SSG mode. However, unlike the sample B using the SSG-DBR laser of the prior art shown in FIG. 12, no significant decrease in the optical output on the short wavelength side is observed, and a uniform optical output is obtained over the entire wavelength-tunable range, as in Example 1. The wavelength of the channel where the maximum optical output was obtained was 1300 nm and the optical output at the time of modulation was 6.3 dBm. The channel having the minimum optical output had a wavelength of 1295 nm, and the optical output at the time of modulation was 0.6 dBm. The fluctuation width of the entire optical output was 5.7 dB at maximum, and compared with the fluctuation width of 12.1 dB in the configuration of the prior art shown in FIG. 11, the wavelength dependency of the optical output was greatly improved.

[0094] In each of the examples described above, the wavelength-tunable optical transmitter has been described assuming that SOAs are also integrated. However, a wavelength-tunable optical transmitter having a configuration in which only a wavelength-tunable DBR laser and an EA modulator are integrated without including SOA exhibits the same effect as in the examples, and the wavelength dependency of the final optical output from the EA modulator is improved.

[0095] As described in detail above, in the wavelength-tunable optical transmitter of the present disclosure, the diffraction grating of the SSG-DBR is set to a configuration different from that of the prior art so that oscillation occurs at the reflection peak of the shortest wavelength in the absence of a DBR current. Thus, a flat optical output characteristic in which the wavelength dependency of the final optical output is suppressed is realized.

INDUSTRIAL APPLICABILITY

[0096] The present invention can be applied to a communication device in an optical communication system.