Wavelength tunable light source, optical transmission apparatus using the same, and method of controlling wavelength tunable light source

A wavelength tunable light source includes: a common wavelength filter that has periodic transmission peak wavelengths or reflection peak wavelengths and is commonly used for a plurality of channels; a wavelength tunable filter that is coupled to the common wavelength filter and has a one-input and multiple-output configuration which has a plurality of output ports, and that has a plurality of transmission peak wavelengths corresponding to the plurality of channels at the plurality of output ports; and a plurality of gain media optically coupled to the plurality of output ports of the wavelength tunable filter, wherein a plurality of laser cavities that perform laser oscillation at a plurality of different wavelengths are formed between the common wavelength filter and the plurality of gain media.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2019-193560, filed on Oct. 24, 2019, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a wavelength tunable light source, an optical transmission apparatus using the same, and a method of controlling the wavelength tunable light source.

BACKGROUND

In order to increase the communication capacity, wavelength division multiplexing optical communication is performed. In particular, in dense wavelength division multiplexing (DWDM) in which a large number of wavelengths are multiplexed at narrow wavelength intervals, optical signals of several tens of wavelengths or more are multiplexed and transmitted at intervals of 50 GHz. In order to further increase the communication capacity, a DWDM system with a wavelength interval of 25 GHz has also been studied. In order to realize this, a light source with a large number of output wavelengths that are tunable along strict wavelength intervals is requested.

Related art is disclosed in see International Publication Pamphlet No. WO 2016/042658).

SUMMARY

According to an aspect of the embodiments, a wavelength tunable light source includes: a common wavelength filter that has periodic transmission peak wavelengths or reflection peak wavelengths and is commonly used for a plurality of channels; a wavelength tunable filter that is coupled to the common wavelength filter and has a one-input and multiple-output configuration which has a plurality of output ports, and that has a plurality of transmission peak wavelengths corresponding to the plurality of channels at the plurality of output ports; and a plurality of gain media optically coupled to the plurality of output ports of the wavelength tunable filter, wherein a plurality of laser cavities that perform laser oscillation at a plurality of different wavelengths are formed between the common wavelength filter and the plurality of gain media.

DESCRIPTION OF EMBODIMENTS

In a four-wavelength laser device, a configuration using first to fourth individual wavelength selection filters and a fifth wavelength selection filter commonly used for four channels is may be used.

Laser devices individually select wavelengths by using individual ring filters having no correlation between channels. This requests a wavelength monitor or an adjustment mechanism for each channel, which increases the size of the laser device. Since pieces of light of respective wavelengths are amplified and output by individual semiconductor optical amplifiers (SOAs), a coupler for multiplexing the pieces of light of respective wavelengths into one fiber is separately requested. When a simple coupler is used for multiplexing, optical loss occurs. When multiplexing with a WDM coupler, additional wavelength adjustment is requested.

A small-sized wavelength tunable light source capable of outputting light having a large number of wavelengths with a simplified configuration and a control method of a tunable wavelength may be provided.

FIG.1is a schematic diagram of a transmission side of an optical transmission apparatus1using a wavelength tunable light source10of a present embodiment. The optical transmission apparatus1is a DWDM transmission apparatus, and includes the wavelength tunable light source10, a demultiplexer2, an optical modulator array3, and a multiplexer4on the transmission side. The optical transmission apparatus1improves the spectrum utilization efficiency and transmission capacity of a transmitter by using a large number of wavelength channels arranged at high density at wavelength intervals of about a modulation symbol rate.

In an example inFIG.1, the wavelength tunable light source10outputs light of four adjacent channels at 50 GHz intervals, but this is an example, and it is possible to output light of different wavelengths over several tens of channels. The wavelength interval may be 12.5 GHz, 25 GHz, 100 GHz, or the like. In this case, modulation baud rates are 12.5 Gbaud, 25 Gbaud, and 100 Gbaud, respectively.

As will be described later, the wavelength tunable light source10includes a common wavelength filter used in common among a plurality of channels. Oscillation wavelength intervals of a plurality of laser cavities are defined by one common wavelength filter having periodic transmission peaks or reflection peaks, thereby obtaining highly accurate wavelength intervals. For laser resonance at a plurality of wavelengths, one wavelength tunable filter of one-input and multiple-output type coupled to a common wavelength filter is used instead of providing an individual wavelength filter for each of the plurality of channels. As a result, wavelength intervals matched with the period of the transmission (or reflection) peak wavelengths of the common wavelength filter are obtained at the output ports of the wavelength tunable filter.

Optical signals modulated and multiplexed for respective wavelengths are multiplexed in one optical fiber and output to a transmission path.

A part of the output light of the wavelength tunable light source10may be used as local oscillation light for detecting an optical signal on a receiving side of the coherent optical transmission apparatus1.

First Embodiment

FIG.2is a schematic diagram of a wavelength tunable light source10A according to a first embodiment. The wavelength tunable light source10A includes a common wavelength filter11, a wavelength tunable filter12, a gain array13, a second wavelength tunable filter15, and an SOA17for collective amplification. A wavelength adjustment mechanism16may be optically coupled to the common wavelength filter11.

The common wavelength filter11includes, for example, a ring resonator111formed of a Si waveguide, waveguides112and113arranged in the vicinity of the ring resonator111, and an optical coupler114that couples the waveguides112and113.

The ring resonator111has periodically varying peak wavelengths. A peak interval is determined by an effective optical path length (circumference) of the ring resonator111. Light incident on the common wavelength filter11from the wavelength tunable filter12is branched by the optical coupler114and propagates to the waveguides112and113. In the light coupled from the waveguide112to the ring resonator111, the light components that match the peak wavelengths of the ring resonator111circulate in the ring resonator111and thus intensify each other due to interference. The light reaching a certain intensity is coupled to the waveguide113, and enters the wavelength tunable filter12from the optical coupler114. Similarly, in the light coupled from the waveguide113to the ring resonator111, the light components that match the peak wavelengths of the ring resonator111circulate in the ring resonator111and thus intensify each other due to interference. The light reaching a certain intensity is coupled to the waveguide112, and enters the wavelength tunable filter12from the optical coupler114.

The wavelength tunable filter12has a one-input and multiple-output configuration. There is one port on a side adjacent to the common wavelength filter11and a plurality of ports on an opposite side. In this example, the wavelength tunable filter12is, for example, a filter in which Mach-Zehnder (MZ) interferometer waveguides formed of Si waveguides are coupled in a multi-stage tree shape. InFIG.2, for convenience of illustration, each MZ interferometer is illustrated as having a pair of waveguides arranged symmetrically, but the wavelength tunable filter12is configured by asymmetric Mach-Zehnder interferometer (AMZI) waveguides.

FIG.3illustrates a specific configuration example of the wavelength tunable filter12inFIG.2. The lengths of the two waveguides (arms) are different in each of the plurality of AMZIs arranged so as to form a three-stage branch waveguide. The two pieces of light propagating through the two arms are multiplexed after being subjected to phase changes corresponding to effective optical path lengths of the respective arms (physical length of waveguide×effective refractive index). The optical output of the AMZI having an effective optical path length difference has periodic dependence on a reciprocal of the wavelength, and a desired wavelength interval may be designed by designing the arm length difference.

By providing a phase shifter PS such as a heater, electrodes, or the like in each arm of each AMZI, the refractive index of the waveguide may be adjusted to finely adjust the wavelength.

In a case of a multi-stage wavelength tunable filter, the number of AMZIs used in each stage increases by a power of two. In the three-stage configuration, the number of output ports of the wavelength tunable filter12is 23=8 ports. Here, an example is used in which eight pieces of light of wavelengths of λ1to λ8are taken out from eight output ports, but the number of stages may be designed according to the number of wavelengths to be multiplexed.

As will be described later, the wavelength tunable filter12is not limited to the AMZI waveguide configuration, and various configurations such as a ring resonator type, a distributed feedback type, an arrayed waveguide type, and the like may be adopted.

Returning toFIG.2, the gain array13in which a plurality of gain waveguides are formed is provided on a multiple-output side of the wavelength tunable filter12. The gain array13is, for example, an SOA array formed of a compound semiconductor. The respective gain waveguides of the gain array13are gain media131to138provided individually at the output ports (for example, eight channels) of the wavelength tunable filter12.

An anti-reflection (AR) film13ais formed on an end face on an input side of the gain array13, for example, an end face adjacent to the output port of the wavelength tunable filter12. A low reflection (LR) film13bis formed on an end face opposite to the face on which the anti-reflection film13ais formed.

Between the ring resonator111and the respective gain media131to138, respective laser cavities that perform laser oscillation at different wavelengths are formed. The light travels back and forth between the low reflection film13bof each of the gain media131to138and the ring resonator111, and a part of the light amplified due to stimulated emission is taken out from the low reflection film13b. In an example inFIG.2, eight pieces of light of different wavelengths λ1to λ8corresponding to the eight channels (seeFIG.3) are taken out from the gain array13.

The gain media131to138are optically coupled to the second wavelength tunable filter15at the end faces on the output side on which the low reflection films13bare provided. Similarly to the wavelength tunable filter12, the second wavelength tunable filter15has the configuration of one-input and multiple-output (or multiple-input and one-output). The second wavelength tunable filter functions as a wavelength selection filter and at the same time functions as a multiplexer. At an output end of the second wavelength tunable filter15, the eight pieces of light of different wavelengths are multiplexed and output.

The optical amplifier17is coupled to the output of the second wavelength tunable filter15functioning as the multiplexer. The optical amplifier17is, for example, a booster SOA formed of the compound semiconductor. An anti-reflection film17ais formed on an incident side end face of the optical amplifier17, and an anti-reflection film17bis formed on an emission side end face.

The optical amplifier17collectively amplifies the light having the different multiple wavelengths. Thus, high optical output and power efficiency are realized. Four-wave mixing (FWM) in the SOA increases the number of output wavelengths. Additional wavelength channels28and29generated by FWM are generated, for example, on a low frequency side and a high frequency side of the wavelength band including the original eight wavelengths.

InFIG.2, the one-input and multiple-output wavelength tunable filter12is coupled to the common wavelength filter11, and the gain array13having the plurality of gain media131to138is optically coupled to the output ports of the wavelength tunable filter12. The number and size of the wavelength filters and the size of the gain medium may be reduced, and thus the wavelength tunable light source10A may be downsized as a whole. When the optical amplifier17is used as the booster SOA, the number of wavelengths may be increased.

In the wavelength tunable filter12, the plurality of wavelength filters correlated with each other are formed by the AMZIs coupled in the multi-stage in a tree or a tournament bracket shape. Therefore, it is not requested to monitor or adjust the wavelengths individually for the respective wavelength filters, and the burden of monitoring and fine adjustment for the respective wavelengths is reduced. Wavelength monitoring and control in the wavelength tunable light source of the embodiment will be described later with reference toFIG.6.

FIG.4illustrates transmission spectra of respective filters of the wavelength tunable light source10A inFIG.2. A solid line is a periodic transmission spectrum of the ring resonator111of the common wavelength filter11. Four different broken lines are transmission spectra at the four output ports of the wavelength tunable filter12.

Center wavelengths of the transmission spectra at respective output ports of the wavelength tunable filter12substantially match the periodic peak wavelengths λ1to λ4of the ring resonator111of the common wavelength filter11.

In the configuration inFIG.2, the individual wavelengths are selected by the one wavelength tunable filter12, and are resonated by the one common wavelength filter11having the periodic transmission peaks or reflection peaks. This configuration allows the peak wavelength intervals between the output ports to match the intervals between the peaks of the ring resonator111.

Second Embodiment

FIG.5is a schematic diagram of a wavelength tunable light source10B according to a second embodiment. In the first embodiment, the second wavelength tunable filter15is used on the output side of the gain array13to multiplex the plurality of pieces of light of the wavelengths. In the second embodiment, the plurality of pieces of light of the wavelengths are multiplexed and collectively amplified without using the second wavelength tunable filter. The collective amplification may not be requested, and a configuration that outputs the light of the plurality of wavelengths corresponding to the original channels is possible.

The wavelength tunable light source10B includes the common wavelength filter11, the wavelength tunable filter12, a gain array23, an output tap21provided between the common wavelength filter11and the wavelength tunable filter12, and an optical amplifier27coupled to the output tap21. A wavelength adjustment mechanism16may be optically coupled to the common wavelength filter11.

The common wavelength filter11is the same as the common wavelength filter11of the first embodiment, and includes the ring resonator111formed of the Si waveguide, the waveguides112and113arranged in the vicinity of the ring resonator111, and the optical coupler114that couples the waveguides112and113.

As in the first embodiment, the wavelength tunable filter12has the one-input and multiple-output configuration. There is one port on a side adjacent to the common wavelength filter11and a plurality of ports on an opposite side. In this example, the wavelength tunable filter12is, for example, a filter in which Mach-Zehnder (MZ) interferometer waveguides formed of Si waveguides are coupled in a multi-stage tree shape. InFIG.5, for convenience of illustration, for each MZ interferometer is illustrated as having the pair of waveguides arranged symmetrically, but the wavelength tunable filter12is actually configured by the AMZI waveguides, each having the effective optical path length difference between the two arms, as illustrated inFIG.3.

The gain array23in which a plurality of gain waveguides are formed is provided on the multiple-output side of the wavelength tunable filter12. The gain array23is, for example, the SOA array formed of the compound semiconductor. The respective gain waveguides of the gain array23are gain media231to238provided individually at the output ports (for example, eight channels) of the wavelength tunable filter12.

An anti-reflection film23ais formed on an end face on an input side of the gain array23, for example, an end face adjacent to the output port of the wavelength tunable filter12. Unlike the first embodiment, a high reflection (HR) film23bis formed on an end face opposite to the end face on which the anti-reflection film23ais formed.

Between the ring resonator111and the respective high reflection films23bof the gain media231to238, respective laser cavities that perform laser oscillation at different wavelengths are formed. The light travels back and forth between each of the gain media231to238and the ring resonator111and is amplified due to the stimulated emission. The pieces of light reflected by the high reflection films23bpass through the wavelength tunable filter12along optical paths in the opposite directions of the pieces of light entering the gain array23and are multiplexed at the one port on the input side of the wavelength tunable filter12.

A part of the amplified and multiplexed light (a fixed ratio of the optical power propagating through the waveguide coupling the wavelength tunable filter12and the common wavelength filter11) is taken out from the output tap21between the common wavelength filter11and the wavelength tunable filter12. For example, the light having the different wavelengths λ1to λ8corresponding to the eight channels (seeFIG.3) is taken out from the output tap21.

The light including the plurality of different wavelengths is collectively amplified by the optical amplifier27, which is the booster SOA, and output. An anti-reflection film27aand an anti-reflection film27bare formed on an incident side end face and an emission side end face of the optical amplifier27, respectively.

As described above, the optical amplifier27may not be requested, but by providing the optical amplifier27, the light having the multiple wavelengths is collectively amplified, and high optical output and power efficiency are realized. Four-wave mixing (FWM) in the SOA increases the number of output wavelengths. Additional wavelength channels28and29generated by FWM are generated, for example, on a low frequency side and a high frequency side of the wavelength band including the original eight wavelengths.

In the configuration inFIG.5, the one-input and multiple-output wavelength tunable filter12is coupled to the common wavelength filter11, and the gain array23having the plurality of gain media231to238is optically coupled to the output ports of the wavelength tunable filter12. The plurality of resonant waveguides formed in the AMZI tree structure are folded back at the gain array23, and the overall size of the wavelength tunable light source10B may be further reduced.

When the light having the plurality of wavelengths multiplexed by the wavelength tunable filter12is collectively amplified by the optical amplifier27, which is the booster SOA, the number of wavelengths may be increased by using four-wave mixing in the SOA.

Also in the wavelength tunable light source10B of the second embodiment, as described with reference toFIG.4, the center wavelengths of the transmission spectra at the respective output ports substantially match the periodic peak wavelengths of the ring resonator111of the common wavelength filter11. As a result, highly accurate wavelength intervals may be obtained with the small-sized wavelength tunable light source10B.

FIG.6is a schematic diagram of a wavelength tunable light source10C used for controlling a tunable wavelength. The wavelength tunable light source10C has a configuration folded back at the gain array23as in the second embodiment. A plurality of laser cavities that perform laser oscillation at different wavelengths are formed between the common wavelength filter11and the end faces of the respective gain media231to238of the gain array23on which the high reflection films23bare formed. The wavelength tunable filter12functions as the wavelength selection filter that selects the respective wavelengths and at the same time functions as the multiplexer that multiplexes the pieces of light amplified by the gain media231to238.

A part of the light of respective wavelengths amplified due to the stimulated emission is taken out from the output tap21between the common wavelength filter11and the wavelength tunable filter12, collectively amplified by the optical amplifier27, and output.

In an example inFIG.6, a longitudinal mode adjusting unit170is arranged between the wavelength tunable filter12and the gain array23. Although the longitudinal mode adjusting unit170may not be requested, by inserting the longitudinal mode adjusting unit, each laser resonator may oscillate a laser with a stable single wavelength.

The wavelength tunable light source10C is provided with the wavelength adjustment mechanism16, and wavelength control is performed based on the output of the wavelength adjustment mechanism16. In an example inFIG.6, a part of the light resonated in the ring resonator111of the common wavelength filter11is taken out from the waveguide112and input to the wavelength adjustment mechanism16.

The wavelength adjustment mechanism16includes, for example, a beam splitter161, a photodetector162, a filter163having a periodic transmission spectrum, and a photodetector164. The beam splitter161splits the input light into two. One of the split pieces of light is detected by the photodetector162, and the intensity is monitored. Another of the split pieces of light passes through the filter163and is then detected by the photodetector164.

As the filter163having the periodic transmission spectrum, for example, an etalon filter may be used. By detecting the light intensity through the etalon filter, the oscillation wavelength may be monitored with high accuracy.

The output of the wavelength adjustment mechanism16is supplied to a control mechanism30and used for wavelength control of the wavelength tunable light source10C. The control mechanism30may be realized by a microprocessor with a built-in memory, a logic device such as FPGA, or the like. The control using the wavelength adjustment mechanism16and the control mechanism30may be applied not only to the wavelength tunable light source10C inFIG.6but also to the wavelength tunable light source10A of the first embodiment and the wavelength tunable light source10B of the second embodiment.

The control mechanism30may control the periodic transmission peak wavelength of the ring resonator111of the common wavelength filter11based on the output of the wavelength adjustment mechanism16(control1). The ring resonator111is controlled by controlling a phase shifter115provided in the ring resonator111. As an example, the temperature of the heater functioning as the phase shifter115is controlled to change the local temperature of the Si waveguide forming the ring resonator111, thereby adjusting the effective refractive index sensed by the propagating light.

The control mechanism30controls at least some of the AMZIs31to37forming the wavelength tunable filter12based on the output of the wavelength adjustment mechanism16(control2). The temperature of the wavelength selection waveguide may be controlled using at least some of the phase shifters PS provided in the respective arms of the AMZIs31to37.

In the one-input and multiple-output wavelength tunable filter12of the embodiment, since the formed wavelength selection waveguides are correlated with each other, it may not be requested to control all AMZIs for each wavelength. An example of control of the wavelength tunable filter12will be described later.

The control mechanism30may control a longitudinal mode adjusting unit170based on the output of the wavelength adjustment mechanism16(control3). As an example, the longitudinal mode adjusting unit170includes phase shifters171to178provided in the plurality of output ports of the wavelength tunable filter12, respectively. The longitudinal mode may be adjusted by controlling at least some of the phase shifters171to178.

In the wavelength tunable light sources10A to10C, it may not be requested to perform all of the control1to the control3, and at least one of the control1to the control3may be performed based on the output of the wavelength adjustment mechanism16.

In a state inFIG.6, as an example, the gain medium231of the gain array23is turned on, and wavelength control is performed focusing on a channel using the gain medium231. At this time, the other gain media232to238are turned off. When a next channel is selected for wavelength adjustment, the gain medium of the selected channel is turned on and the other channels are turned off.

After individual channels are controlled, all channels may be controlled. In the one-input and multiple-output wavelength tunable filter12, since the plurality of waveguides for wavelength selection are correlated with each other, a large amount of entire wavelength adjustment is automatically performed when the wavelength adjustment of each channel is completed. When the entire wavelength adjustment is additionally performed in order to further improve the accuracy of the wavelength adjustment, all of the gain media231to238are turned on, and the wavelength tunable filter12is finely adjusted based on the output of the wavelength adjustment mechanism16.

FIG.7is a flowchart of a method of controlling the wavelength tunable light source according to the embodiment. This control flow is executed by the control mechanism30. First, a channel of any one wavelength is activated (S11). For example, any one of the gain media231to238in the laser resonator is turned on to cause laser oscillation at a single wavelength.

Next, while monitoring the output of the wavelength adjustment mechanism16, the ring resonator111of the common wavelength filter11is controlled so that the wavelengths of the periodic transmission peaks or reflection peaks have the desired wavelength intervals (S12).

The wavelength tunable filter12is controlled so that the detection power of the wavelength adjustment mechanism16becomes maximum (S13). The fact that the power of the light detected by the wavelength adjustment mechanism16is maximized means that the peak wavelength of the AMZI filter of the channel of interest matches the peak wavelength of the ring resonator111.

Optionally, the longitudinal mode adjusting unit170is adjusted based on the output of the wavelength adjustment mechanism16(S14). The phase shifter (one of171to178) of the corresponding channel is controlled to finely adjust the phase to a point at which the laser output of the channel is most stabilized against the mode hop phenomenon.

Thereafter, the gain SOA (gain array23) is once turned off (S15). It is determined whether there is another channel to be controlled (S16), and when there is another channel, S11and S13to S16are repeated. For a second and subsequent channels, the control of the ring resonator111of the common wavelength filter11(S12) may be skipped. When there is no other channel to be controlled, all the channels are turned on, the output of the wavelength adjustment mechanism16is monitored again, and the oscillation wavelengths are finely adjusted (S17). Thereafter, the optical amplifier27, which is the booster SOA, is turned on to output the light having multiple wavelengths (S18).

In a loop from S11to S16, the control of the wavelength tunable filters12of the second and subsequent channels (S13) is simpler than the wavelength control of the first channel. Alternatively, control of some channels may be skipped without controlling all of the second and subsequent channels.

For example, inFIG.6, in the control of the first channel, the first stage AMZI31, the second stage AMZI32, and the third stage AMZI34are selected, and the wavelength λ1is adjusted using the phase shifter PS. When the wavelength λ1of the wavelength tunable filter12is adjusted, the wavelength λ2of the adjacent channel is also adjusted substantially accurately.

Accordingly, the adjustment of the second channel is skipped, and the wavelength λ3is adjusted in the third channel. At this time, since the first stage AMZI31and the second stage AMZI32have been accurately adjusted by the channel adjustment of λ1, only the AMZI35of the third stage may be controlled. When the wavelength λ3of the wavelength tunable filter12is adjusted, the wavelength λ4of the adjacent channel is also adjusted substantially accurately. Therefore, the adjustment of the fourth channel may be skipped.

When the wavelength λ5is adjusted in the fifth channel, since the first stage AMZI31has already been adjusted, the second stage AMZI33and the third stage AMZI36are controlled. When the wavelength λ5of the wavelength tunable filter12is adjusted, the wavelength λ6of the adjacent channel is also substantially accurately adjusted, and the adjustment of the sixth channel (wavelength λ6) may be skipped.

When the wavelength λ7is adjusted in the seventh channel, since the first stage AMZI31and the second stage AMZI33have already been adjusted, only the third stage AMZI37may be controlled. When the wavelength λ7of the wavelength tunable filter12is adjusted, the wavelength λ8of the adjacent channel is also adjusted substantially accurately. Therefore, the adjustment of the eighth channel may be skipped.

As described above, by coupling the one-input and multiple-output wavelength tunable filter12to the common wavelength filter11, it is possible to increase the correlation between the respective channels and reduce the burden of adjusting the individual wavelengths.

FIG.8illustrates a wavelength tunable filter22A as a modification of the wavelength tunable filter12. The wavelength tunable filter22A is an array waveguide gratings (AWG) wavelength selection filter. A slab waveguide223is provided on an input side of an arrayed waveguide221formed of a plurality of arrayed waveguides having different effective optical path lengths, and a slab waveguide222is formed on an output side. The slab waveguide223on the input side is coupled to the common wavelength filter11by one input waveguide. The slab waveguide222on the output side couples the pieces of light incident from the plurality of arrayed waveguides to output waveguides having corresponding wavelengths.

In the AWG wavelength tunable filter22A, the pieces of light may be coupled to different ports for respective wavelengths by using wavelength dispersion generated by the pieces of light propagating through a large number of arrayed waveguides having different effective optical path lengths. The wavelength interval may be designed by the lengths of the arrayed waveguides and the positions of the output ports. The pieces of light of wavelengths λ1to λNoutput from the respective output ports of the slab waveguide222are incident on the corresponding gain media. The wavelength tunable filter22A is also the one-input and multiple-output wavelength tunable filter.

FIGS.9A and9Billustrate a structure for adjusting the peak wavelength of the wavelength tunable filter22A inFIG.8. InFIGS.9A and9B, an arrayed waveguide221A and an arrayed waveguide221B are provided with phase controllers224A and224B whose lengths are not uniform, respectively. InFIG.9A, the length of a phase control region is longer in the outer (longer) arrayed waveguide. This configuration may be referred to as a positive filter. InFIG.9B, the length of a phase control region is longer in the inner (shorter) arrayed waveguide. This configuration may be referred to as a negative filter. For example, the voltage applied to the phase controller224A or224B may be controlled to adjust the center peak wavelength of the AWG wavelength tunable filter.

FIG.10illustrates a wavelength tunable filter22B as another modification of the wavelength tunable filter12. The wavelength tunable filter22B is an echelle grating wavelength selection filter. The wavelength tunable filter22B includes one input waveguide225, a slab region228, and N output waveguides227. The input waveguide225and the N output waveguides227are arranged on the same side of the slab region228.

The slab region228has a sidewall diffraction grating229at an end portion opposite to the input/output waveguides. In the sidewall diffraction grating229, pieces of light of a plurality of orders (for example, zeroth to mth orders, m=3 in an example inFIG.10) is diffracted in the same direction. The light emission position is changed for each wavelength by the wavelength dispersion effect of the sidewall diffraction grating229, and the pieces of light of different wavelengths λ1to λNare output for respective ports.

The peak wavelength may be finely adjusted by providing a wavelength control region226in the slab region228and changing the refractive index by temperature control or the like. The wavelength tunable filter22B is also the one-input and multiple-output wavelength selection filter, and each of the plurality of output waveguides227is coupled to the corresponding gain medium.

Through the embodiments and the modifications, the wavelength tunable filter coupled to the common wavelength filter11has the one-input and multiple-output configuration and has the transmission peak wavelengths periodically arranged for the respective output ports. The oscillation wavelengths and intensities of the multiple channels are monitored by the common wavelength adjustment mechanism16and controlled by the common control mechanism30. It is possible to achieve both multi-channel and miniaturization while maintaining the strict oscillation wavelength intervals by the common wavelength filter11.

The present embodiments are not limited to the above-described configuration examples, and include various modifications and alternatives. For example, the resonator having the periodic transmission or reflection peaks of the common wavelength filter11is not limited to the ring resonator, and may be a racetrack type, a double ring type, an elliptical type, or the like. The wavelength tunable filter having the plurality of transmission peak wavelengths may be a ring resonator type instead of the AMZI type or the AWG type. In either case, a small-sized wavelength tunable light source maintaining strict wavelength intervals may be obtained.

In addition to the above description, the following appendices are presented.