JP6038059B2 - Wavelength variable light source and wavelength variable light source module - Google Patents

Description

  The present invention relates to a wavelength variable light source in which a plurality of semiconductor lasers are integrated and a wavelength variable light source module including the wavelength variable light source.

  2. Description of the Related Art In recent years, with a dramatic increase in communication demand, a wavelength division multiplexing communication system has been realized that enables large-capacity transmission with a single optical fiber by multiplexing a plurality of signal lights having different wavelengths.

  As a light source used in the wavelength division multiplexing communication system, a single mode LD (Laser Diode) (hereinafter referred to as a single diode) capable of obtaining a high side mode suppression ratio (SMSR) of at least 30 to 40 dB or more. (Referred to as mode LD). Examples of the single mode LD include a distributed feedback laser diode (hereinafter referred to as DFB-LD) and a distributed Bragg reflector laser diode (hereinafter referred to as DBR-LD). .

  In order to realize a wavelength division multiplexing communication system, a low-cost tunable light source that covers the entire wavelength band is required. As the wavelength tunable light source, an LD light source monolithically integrated on the same substrate has attracted attention.

  In general, the above-mentioned wavelength tunable light source is used in combination with an external modulator module that generates a data signal. In addition, an electroabsorption (EA) type optical modulator or a Mach Zehnder (MZ) type is used. Research is also underway to monolithically integrate optical modulators on the same substrate.

  Conventionally, the output side of a plurality of DFB-LDs is connected to an input waveguide of a multimode interference (Multi Mode Interference) type optical multiplexing circuit (hereinafter referred to as MMI), and the light multiplexed by the MMI is a semiconductor optical amplifier. A variable wavelength light source configured to output from an output waveguide after being amplified by a semiconductor optical amplifier (hereinafter referred to as SOA) is disclosed (for example, see Patent Document 1). In the following, the MMI of K input (K is a natural number) and L output (L is a natural number) will be referred to as K × L-MMI. That is, the K input refers to K input waveguides connected to the MMI, and the L output refers to L output waveguides connected to the MMI.

  In addition, a wavelength tunable light source in which the output sides of a plurality of LDs are connected to N × 2-MMI (N is a natural number of 3 or more), and two output waveguides of MMI are connected to two arms of a Mach-Zehnder modulator. It is disclosed (for example, see Patent Document 2). According to Patent Document 2, it is possible to improve the optical loss as compared with the case of using N × 1-MMI.

  Further, the output sides of the plurality of DFB-LDs are connected to the MMI, and after phase adjustment is performed in each of the two output waveguides of the MMI, light is output from one output waveguide via the other MMI. A tunable light source configured to be configured is disclosed (for example, see Patent Document 3). According to Patent Document 3, it is possible to increase the output and improve the SN ratio (Signal to Noise ratio).

JP 2003-258368 A JP 2007-65357 A JP 2011-44581 A

  In a wavelength division multiplexing communication system having a transmission rate of 40 Gbps or higher in a trunk line system, digital coherent communication using optical phase modulation has recently been put into practical use. In a wavelength division multiplex communication system employing digital coherent communication, a narrow linewidth variable wavelength light source having a laser oscillation line width of 1 MHz or less, more preferably 500 kHz or less is used as a light source for transmission and reception.

  In Patent Document 1, when a current is applied to the SOA to output light (when the SOA is turned on), the heat generated in the SOA reaches the DFB-LD and is affected by the heat. There is a problem that the oscillation wavelength of the LD changes. For example, a module equipped with an element such as DFB-LD is equipped with a wavelength monitor (monitor for detecting the wavelength of light output from the DFB-LD) so that the oscillation wavelength of the DFB-LD is always constant. Even if it is controlled, the oscillation wavelength of the DFB-LD fluctuates due to the influence of heat generated at the moment the current is applied to the SOA, so the time until the oscillation wavelength of the DFB-LD stabilizes (startup time) Needed.

  In addition, in the wavelength division multiplex communication employing the above digital coherent communication, if the wavelengths of the optical signals used for transmission and reception are different, separate wavelength variable light sources are required for transmission and reception. The power consumption of the entire transmission / reception apparatus including the wavelength tunable light source increases.

  On the other hand, in the wavelength division multiplex communication employing the above-mentioned digital coherent communication, there are many cases where the wavelengths of optical signals used for transmission and reception are the same, which is generally required for transmission and reception. The light output of the line width variable wavelength light source is different. In such a case, it is desirable to use one tunable light source as a light source for transmission and reception from the viewpoint of reducing power consumption of the entire transmission and reception device and reducing the mounting area.

  For example, in the case where the wavelength tunable light source disclosed in Patent Documents 1 to 3 is also used as a light source for transmission and reception, the wavelength tunable light source disclosed in Patent Documents 1 to 3 needs to have two outputs by a polarization maintaining coupler. However, in such a configuration, a branching loss occurs in the polarization maintaining coupler. Therefore, in order to compensate for the branching loss, the injection current to the SOA provided on the output side of the wavelength tunable light source is increased to increase the light from the wavelength tunable light source. Although it is necessary to increase the output, there is a problem that it causes an increase in power consumption and laser oscillation line width.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a wavelength tunable light source and a wave light tunable light source module capable of stabilizing by suppressing fluctuations in oscillation wavelength. .

In order to solve the above problems, a wavelength tunable light source according to the present invention has an input side and an output side, one end of a plurality of input waveguides is connected to the input side, and at least one or more output guides are connected to the output side. A waveguide is connected, and the light input from each input waveguide is multiplexed, and an optical multiplexing circuit that outputs the combined light to the output waveguide; and the other end of each input waveguide; A plurality of semiconductor lasers capable of single mode oscillation at different wavelengths, a pre-stage optical amplifier connected to the optical multiplexing circuit side of the output waveguide, and a post-stage optical amplifier connected to the post-stage side of the pre-stage optical amplifier of the output waveguide; ON / OFF of the light output from the output waveguide is controlled by the post-stage optical amplifier, and the light output from the output waveguide is consumed by the pre-stage optical amplifier and the post-stage optical amplifier when the output is ON. Power is output from the output waveguide That the output of the light so that equal power dissipated in the OFF at front-stage optical amplifier and the rear-stage optical amplifier, characterized that you control the front-stage optical amplifier and the rear-stage optical amplifier.

According to the present invention, it has an input side and an output side, one end of a plurality of input waveguides is connected to the input side, at least one or more output waveguides are connected to the output side, and input from each input waveguide And a plurality of semiconductors that are connected to the other end of each input waveguide and that can oscillate in a single mode at different wavelengths. A laser, a front-stage optical amplifier connected to the optical multiplexing circuit side of the output waveguide, and a rear-stage optical amplifier connected to the rear stage side of the front-stage optical amplifier of the output waveguide, and light output from the output waveguide is The ON / OFF of the output is controlled by the post-stage optical amplifier, and the power consumed by the pre-stage optical amplifier and the post-stage optical amplifier when the output of the light output from the output waveguide is ON, the light output from the output waveguide When the output is OFF, the pre-stage optical amplifier and As equal power consumed by the rear-stage optical amplifier, order to control the front-stage optical amplifier and the rear-stage optical amplifier, it is possible to stabilize and inhibit the fluctuation of the oscillation wavelength.

It is a figure which shows an example of a structure of the wavelength variable light source by Embodiment 1 of this invention. It is a figure which shows an example of the fluctuation | variation of the oscillation wavelength of a wavelength variable light source at the time of switching the optical output by Embodiment 1 of this invention from OFF. It is a figure for demonstrating an example of control of the front | former stage SOA and back | latter stage SOA of the wavelength variable light source by Embodiment 2 of this invention. It is a figure for demonstrating an example of control of the front | former stage SOA and back | latter stage SOA of the wavelength variable light source by Embodiment 2 of this invention. It is a figure which shows an example of the fluctuation | variation of the oscillation wavelength of a wavelength variable light source at the time of switching the optical output by Embodiment 2 of this invention from OFF. It is a figure which shows an example of a structure of the wavelength variable light source by Embodiment 3 of this invention. It is a figure which shows an example of the fluctuation | variation of the oscillation wavelength of the optical output for transmission at the time of switching the optical output for reception by Embodiment 3 of this invention from OFF to ON. It is a figure which shows another example of a structure of the wavelength variable light source by Embodiment 3 of this invention. It is a figure which shows an example of the fluctuation | variation of the oscillation wavelength of the optical output for transmission at the time of switching the optical output for reception by Embodiment 3 of this invention from OFF to ON. It is a figure which shows an example of a structure of the wavelength variable light source module by Embodiment 4 of this invention. It is a figure which shows an example of a structure of the transmission / reception apparatus by Embodiment 5 of this invention. It is a figure which shows an example of a structure of the wavelength variable light source by a prerequisite technique. It is sectional drawing which shows an example of a structure of the MMI input waveguide in the wavelength variable light source by a premise technique. It is sectional drawing which shows an example of a structure of SOA in the wavelength tunable light source by a base technology. It is a figure which shows an example of a structure of the transmitter / receiver by a premise technique. It is a figure which shows an example of a structure of the wavelength variable light source module by a prerequisite technique.

  Embodiments of the present invention will be described below with reference to the drawings.

  In the following, in each figure, the same or similar components are denoted by the same reference numerals or names, and their functions are also the same.

  In addition, the dimensions, materials, shapes, and relative arrangements of the constituent elements exemplified in each embodiment are appropriately changed depending on the configuration of the apparatus to which the present invention is applied and various conditions. The present invention is not limited to these examples.

<Prerequisite technology>
First, the prerequisite technology of the present invention will be described.

  FIG. 12 is a diagram showing an example of the configuration of a wavelength tunable light source according to the base technology, and shows an example of the configuration of a wavelength tunable light source using a plurality of single mode LDs having different oscillation wavelengths.

  The DFB-LD array 2 includes N DFB-LDs 1 and is connected to N × 1-MMI 3 (N is a natural number of 3 or more) MMI input waveguides 4.

  The SOA 6 is connected to the MMI output waveguide 5 of N × 1-MMI3.

  In the above configuration, when an arbitrary DFB-LD1 in the DFB-LD array 2 is laser-oscillated, 1 / N of light output from the DFB-LD1 (hereinafter referred to as LD output light) is MMI output waveguide 5 And the remaining (N−1) / N is radiated out of the MMI output waveguide 5. Compensation for branch loss, coupling loss, and the like is performed by injecting current into the SOA 6, and a high transmission optical output 7 is output from the SOA 6 to the outside.

  FIG. 13 is a cross-sectional view showing an example of the AA cross section of FIG. 12, and shows an example of the configuration of the MMI input waveguide 4.

  The MMI input waveguide 4 is formed by sequentially laminating an InP lower cladding layer 41, an InP current blocking layer 44, an InGaAsP waveguide layer 42, and an InP upper cladding layer 43 on an InP substrate 40. The InGaAsP waveguide layer 42 is made of an InGaAsP-based material that does not absorb LD output light. The InGaAsP waveguide layer 42 may be a bulk epitaxial layer or a multiple quantum well (MQW) layer.

  Although FIG. 13 shows the configuration of the MMI input waveguide 4, the configuration of the MMI output waveguide 5 is the same.

  Further, in the configuration in the wide multimode region of N × 1-MMI3, the width of the InGaAsP waveguide layer 42 (width in the horizontal direction in FIG. 13) is wider than that of the MMI input waveguide 4 and the MMI output waveguide 5. In other words, the configuration is the same as that of the MMI input waveguide 4 and the MMI output waveguide 5.

  FIG. 14 is a cross-sectional view showing an example of the BB cross section of FIG. 12, and shows an example of the configuration of the SOA 6.

  The SOA 6 is formed by sequentially laminating an InP lower clad layer 41, an InP current blocking layer 44 and an InGaAsP active layer 45, an InP upper clad layer 43, an InGaAsP contact layer 46, and an electrode 47 on an InP substrate 40. For the InGaAsP active layer 45, an InGaAsP material having a gain with respect to light passing through the MMI output waveguide 5 (hereinafter referred to as guided light) is used. The InGaAsP active layer 45 may be a bulk epitaxial layer or a multiple quantum well (MQW) layer.

  In FIG. 14, the configuration of the SOA 6 is shown, but the configuration of the DFB-LD 1 is the same. In the DFB-LD 1 and the SOA 6, when current is injected through the electrodes 47 provided on the InP substrate 40 and the InGaAsP contact layer 46, a gain is generated in the InGaAsP active layer 45 and spontaneous emission light is generated. In the DFB-LD1, spontaneous emission light having a specific wavelength reflected by the diffraction grating becomes stimulated emission seed light, and laser oscillation occurs when a predetermined threshold current is exceeded. On the other hand, the SOA 6 functions as an amplifier for the LD output light, but is designed not to oscillate alone.

  The oscillation wavelength of the DFB-LD1 changes at a rate of about 0.1 nm / ° C. according to the temperature of the DFB-LD1 (hereinafter referred to as element temperature). Therefore, when the element temperature is changed in a predetermined range (for example, 10 to 50 ° C.), the oscillation wavelength of any DFB-LD 1 in the DFB-LD array 2 (for example, N = 10 to 16) is The interval of the oscillation wavelength of each DFB-LD1 is designed so as to coincide with the oscillation wavelength of another adjacent DFB-LD1. At this time, the combination of the selection of the DFB-LD1 and the adjustment of the element temperature allows the DFB-LD array 2 formed in one chip to use the entire wavelength band (about 30 to about C band (Conventional band) or L band (Long band)). 40 nm).

  FIG. 15 is a diagram illustrating an example of the configuration of the transmission / reception device 8 according to the base technology, and illustrates the configuration of the transmission / reception device 8 for a digital coherent communication system.

  The transmission light output 7 output from the wavelength variable light source module 9 is modulated by the modulator module 10 and then output to the outside as a transmission signal 11.

  The reception signal 12 received from the outside is input to the reception module 14 together with the reception light output 13 output from the wavelength variable light source module 9, and is restored after the signal processing.

  In the transmission / reception apparatus 8 shown in FIG. 15, since insertion loss occurs in the modulator module 10, the transmission optical output 7 generally requires a high output, but the reception optical output 13 has a relatively low output. There may be.

  However, as described above, when the wavelengths of optical signals used for transmission and reception are the same, one wavelength variable light source module is used for transmission / reception from the viewpoint of reducing the power consumption of the entire transmission / reception device 8 and the mounting area. It is desirable to also use as a light source.

  FIG. 16 is a diagram illustrating an example of the configuration of the wavelength tunable light source module 9 according to the base technology. The wavelengths of optical signals used for transmission and reception are the same, and one wavelength tunable light source module is used as a light source for transmission and reception. Is shown as a configuration.

  The wavelength tunable light source module 9 includes a wavelength tunable light source 15, a coupling optical system 16, a beam splitter 18, and a monitor 19. An optical fiber 17 and a polarization maintaining coupler 20 are connected to the LD output side of the wavelength tunable light source module 9.

  The variable wavelength light source 15 emits single mode LD output light, and the emitted LD output light is coupled to an optical fiber 17 via a coupling optical system 16 including a lens, an optical isolator (not shown), and the like. The LD output light coupled to the optical fiber 17 is branched at a predetermined ratio by the polarization maintaining coupler 20, and each of the branched LD output lights is output as a transmission light output 7 and a reception light output 13.

  Further, part of the LD output light that passes through the coupling optical system 16 is extracted by the beam splitter 18, and the wavelength and output level of the LD output light by the monitor 19 including a wavelength filter and a photodiode (not shown). Is detected.

  In order to output light when a tunable light source with one output (outputs one light) as shown in FIG. 12 is applied to the transmitting / receiving device 8 shown in FIG. 15 or the tunable light source module 9 shown in FIG. In addition, when a current is applied to the SOA 6 (SOA 6 is turned on), heat generated in the SOA 6 reaches the DFB-LD 1 and the oscillation wavelength of the DFB-LD 1 changes.

  For example, when controlling the temperature to be constant, even if a thermistor for detecting the temperature is arranged in the vicinity of the DFB-LD1, the temperature of the DFB-LD1 itself cannot be detected. The oscillation wavelength of the DFB-LD1 varies under the influence of heat.

  Even if a module equipped with an element such as DFB-LD1 is equipped with a wavelength monitor and controlled so that the oscillation wavelength of DFB-LD1 is always constant, the influence of heat generated at the moment when current is applied to SOA 6 In response to this, the oscillation wavelength of the DFB-LD1 fluctuates, and thus it takes time (rise time) until the oscillation wavelength of the DFB-LD1 is stabilized.

  Further, when the polarization maintaining coupler 20 has two outputs (outputs two lights), a branching loss occurs in the polarization maintaining coupler 20, and therefore the output side of the wavelength tunable light source to compensate for the branching loss. Although it is necessary to increase the light output from the wavelength tunable light source by increasing the injection current for the SOA 6 provided in the semiconductor device, there is a problem that the power consumption and the laser oscillation line width are increased.

  The present invention has been made to solve such problems, and will be described in detail below.

<Embodiment 1>
FIG. 1 is a diagram showing an example of the configuration of a wavelength tunable light source according to Embodiment 1 of the present invention.

  The wavelength tunable light source according to the first embodiment of the present invention includes a DFB-LD array 2 composed of 12 DFB-LD1s (semiconductor lasers), an MMI input waveguide 4, and a 12 × 1-MMI 21 (optical multiplexing circuit). ), A front-stage SOA 22 (front-stage optical amplifier), and a rear-stage SOA 23 (rear-stage optical amplifier). The difference from the wavelength tunable light source according to the base technology shown in FIG. 12 is that the front SOA 22 and the rear SOA 23 (two-stage SOA) are connected to the 12 × 1-MMI 21 of the first embodiment.

  In order for the tunable light source shown in FIG. 1 to obtain the same amplification factor and optical output as the tunable light source according to the base technology shown in FIG. 12, the sum of the length of the front-stage SOA 22 and the length of the rear-stage SOA 23 is The length of the SOA 6 (single-stage SOA) shown in FIG. Here, the length of each SOA means the length of the LD output light in the passing direction.

  The 12 × 1-MMI 21 has an input side and an output side, one end of twelve MMI input waveguides 4 is connected to the input side, one MMI output waveguide 5 is connected to the output side, and each The LD output light input from the MMI input waveguide 4 is multiplexed, and the combined LD output light is output to the MMI output waveguide 5.

  The twelve DFB-LDs 1 are connected to the other end of each MMI input waveguide 4 and can each perform single mode oscillation at different wavelengths.

  The front-stage SOA 22 and the rear-stage SOA 23 are serially connected to the MMI output waveguide 5. That is, the front stage SOA 22 is connected to the 12 × 1-MMI 21 side of the MMI output waveguide 5, and the rear stage SOA 23 is connected to the rear stage side of the MMI output waveguide 5, the side of the front stage SOA 22 (the side from which the optical output 25 is output). . An SOA connection waveguide 24 is provided between the front stage SOA 22 and the rear stage SOA 23.

  On the exit end side of the MMI output waveguide 5, waveguide-type SOAs (front SOA 22 and rear SOA 23) having different lengths are provided, and electrodes 47 are independently connected to the front SOA 22 and the rear SOA 23, respectively. (See FIG. 14).

  The pre-stage SOA 22 and the post-stage SOA 23 are formed by removing predetermined portions of the MMI output waveguide 5 and the SOA connection waveguide 24 by etching, and then performing a regrowth technique called butt joint growth, so that the MMI output waveguide 5 and the SOA connection waveguide 24 And the cross sections of the front SOA 22 and the rear SOA 23 are directly joined. Further, in order to suppress the generation of reflected return light at the output end, a non-reflective coating (not shown) is applied to the output end face. By suppressing the generation of reflected return light, an increase in the laser oscillation line width can be suppressed.

  The configurations of the MMI input waveguide 4, the MMI output waveguide 5, and the SOA connection waveguide 24 are the same as the configurations shown in FIG. 13, and the configurations of the DFB-LD1, the front stage SOA 22, and the rear stage SOA 23 are shown in FIG. Since the configuration is the same, the description thereof is omitted here.

  Next, the operation of the wavelength tunable light source will be described.

  When an arbitrary DFB-LD1 is selected and current injection greater than or equal to the threshold current is performed, laser oscillation occurs in the selected DFB-LD1. The LD output light output from the DFB-LD 1 is input to the multi-mode region of 12 × 1-MMI 21 through the MMI input waveguide 4. At this time, if the 12 × 1-MMI 21 is appropriately designed, the LD output light can be coupled to the MMI output waveguide 5 at a ratio of about 1/12 over the entire wavelength band.

  When current is injected into the front stage SOA 22 and the rear stage SOA 23, the LD output light traveling through the MMI output waveguide 5 is amplified by the front stage SOA 22, and the LD output light amplified by the front stage SOA 22 is further amplified by the rear stage SOA 23. Output as an optical output 25.

  However, when the current is injected into the front stage SOA 22 and the current is not injected into the rear stage SOA 23, the LD output light is absorbed by the rear stage SOA 23 and the optical output 25 is not output. As described above, the post-stage SOA 23 has a function (shutter function) for turning on / off the LD light output in addition to the function for amplifying the LD output light. In other words, the output (light output 25) of light output from the MMI output waveguide 5 is controlled to be turned ON / OFF by the rear stage SOA 23.

  Here, the principle of light amplification and attenuation in the SOA will be briefly described.

  When a large amount of current is supplied to the SOA, there are many electrons at a level where the energy in the SOA is high. In this case, an electron transits to a lower vacant level (a level where energy is low and no electron exists) by interaction with light, and energy corresponding to the difference between the two levels is emitted as light. Thereby, light (for example, said LD output light) is amplified and output.

  On the other hand, when the current supplied to the SOA is small, there are many electrons at a level where the energy in the SOA is low. In this case, electrons transit to a higher vacant level while absorbing energy from light (for example, the above-mentioned LD output light) by interaction with light. Thereby, light (for example, said LD output light) attenuate | damps and is not output.

  As described above, the light is amplified by applying current to the front-stage SOA 22, and the light output from the front-stage SOA 22 is further amplified by the rear-stage SOA 23 by controlling the application of current to the rear-stage SOA 23. The optical output 25 can be output (the optical output 25 is turned on), or the light output from the front-stage SOA 22 is attenuated by the rear-stage SOA 23 so that the optical output 25 is not output (the optical output 25 is turned off). Can be.

  In the first embodiment, regarding the control of the front-stage SOA 22 and the rear-stage SOA 23, current is always applied to the front-stage SOA 22, and current is applied only to the rear-stage SOA 23 depending on whether or not the optical output 25 is output. Control whether to do. In this way, by using the two-stage SOA, it is possible to suppress fluctuations in the amount of heat generated when switching the optical output 25 ON / OFF, and thus it is possible to suppress fluctuations in the wavelength of the DFB-LD1. The start-up time required until the oscillation wavelength of the DFB-LD1 is stabilized can be shortened.

  FIG. 2 is a diagram illustrating an example of fluctuations in the oscillation wavelength of the wavelength tunable light source when the optical output is switched from OFF to ON. In FIG. 2, the horizontal axis indicates time, and the vertical axis indicates the shift (variation) in the oscillation wavelength of the wavelength tunable light source (DFB-LD). A broken line indicates a case of a single-stage SOA (see FIG. 12), and a solid line indicates a case of a two-stage SOA (see FIG. 1).

  In the case of the one-stage SOA and the two-stage SOA, any selected DFB-LD is always in the ON state when the optical output is OFF and ON (the LD output light is output from the DFB-LD). Have been). In the case of a two-stage SOA, the front SOA (previous SOA 22) is always ON. When outputting the optical output, the SOA (SOA 6) is turned on in the case of the one-stage SOA, and the rear-stage SOA (rear-stage SOA 23) is turned on in the case of the two-stage SOA.

  Further, the sum of the length of the front SOA and the length of the rear SOA in the two-stage SOA is assumed to be the same as the SOA in the first-stage SOA. It is assumed that the optical output 25 is the same for both the single-stage SOA and the two-stage SOA.

  As shown in FIG. 2, the fluctuation of the oscillation wavelength is suppressed in the case of the two-stage SOA as compared with the case of the single-stage SOA.

  Even if the oscillation wavelength is detected by a wavelength monitor and the temperature is controlled by a Peltier element or the like, the oscillation wavelength is fed back by switching the optical output ON / OFF. Since the temperature of the determined DFB-LD varies, the oscillation wavelength also varies. However, the change in heat quantity is reduced by switching the optical output ON / OFF only with the latter stage SOA of the two-stage SOA, and the temperature fluctuation of the DFB-LD (that is, fluctuation of the oscillation wavelength of the DFB-LD) is suppressed. It is possible to shorten the start-up time until the oscillation wavelength is stabilized.

  In the two-stage SOA, when the sum of the length of the front-stage SOA and the length of the rear-stage SOA is the same as the length of the single-stage SOA, the length of the front-stage SOA is longer and the length of the rear-stage SOA is shorter. Variations in the amount of heat (oscillation wavelength variation) when switching on / off the optical output can be suppressed, and the start-up time can be shortened. However, if the length of the post-stage SOA is made too short, the optical output cannot be completely blocked when the optical output is turned off (when the post-stage SOA is turned off). Therefore, it is necessary to adjust the length of the subsequent SOA as necessary (according to the required amount of cutoff).

  From the above, according to the first embodiment, the fluctuation of the oscillation wavelength of the DFB-LD that occurs when the optical output is switched ON / OFF is suppressed, and the startup time until the oscillation wavelength is stabilized is shortened. can do.

  By applying the wavelength tunable light source according to the first embodiment to the transmission / reception device 8 shown in FIG. 15 or the wavelength tunable light source module 9 shown in FIG. 16, the start-up time until the oscillation wavelength is stabilized is shortened. The transmitting / receiving device 8 or the wavelength tunable light source module 9 can be realized.

<Embodiment 2>
In the first embodiment, a two-stage SOA is used, and ON / OFF of the optical output is switched only by the subsequent SOA, thereby suppressing fluctuations in the oscillation wavelength of the DFB-LD. The second embodiment of the present invention is characterized in that the fluctuation of the oscillation wavelength is further suppressed by controlling not only the rear stage SOA but also the front stage SOA.

  Since the configuration of the wavelength tunable light source according to the second embodiment is the same as that of the first embodiment (see FIG. 1), description thereof is omitted here.

  In the first embodiment, ON / OFF of the optical output 25 is switched only by the rear stage SOA 23, but the fluctuation of the oscillation wavelength of the DFB-LD1 accompanying the change in the amount of heat generated from the rear stage SOA 23 remains.

  FIGS. 3 and 4 are diagrams for explaining an example of control of the front-stage SOA 22 and the rear-stage SOA 23 of the wavelength tunable light source according to the second embodiment. 3 and 4, the horizontal axis indicates the positional relationship between the front-stage SOA 22 and the rear-stage SOA 23 (the same positional relationship as in FIG. 1), and the vertical axis indicates the power applied to the front-stage SOA 22 and the rear-stage SOA 23. FIG. 3 shows a case where the optical output 25 is OFF, and FIG. 4 shows a case where the optical output 25 is ON.

  As shown in FIG. 3, when the optical output 25 is OFF, only the front SOA 22 is ON.

  As shown in FIG. 4, when the optical output 25 is ON, the front-stage SOA 22 and the rear-stage SOA 23 are turned on. At this time, the current (that is, power) applied to the front-stage SOA 22 is slightly reduced as compared with the case where the optical output 25 is OFF (see FIG. 3), and the state is maintained during the ON time. In other words, the current applied to the front stage SOA 22 when ON is controlled to be smaller than the current applied when OFF.

  In the above description, the total power applied to the front-stage SOA 22 and the rear-stage SOA 23 when the optical output 25 is OFF is substantially the same as the total power applied to the front-stage SOA 22 and the rear-stage SOA 23 when the optical output 25 is ON. It is set to be. That is, the power consumed by the front SOA 22 and the rear SOA 23 when the optical output 25 output from the MMI output waveguide 5 is ON, and the front SOA 22 and the rear stage when the output of the optical output 25 output from the MMI output waveguide 5 is OFF. The front-stage SOA 22 and the rear-stage SOA 23 are controlled so as to be equal to the power consumed by the SOA 23.

  FIG. 5 is a diagram illustrating an example of fluctuations in the oscillation wavelength of the wavelength tunable light source when the optical output 25 is switched from OFF to ON. In FIG. 5, the horizontal axis indicates time, and the vertical axis indicates the shift (variation) in the oscillation wavelength of the wavelength tunable light source (DFB-LD). A broken line indicates a case where the preceding SOA is not controlled (Embodiment 1), and a solid line indicates a case where the preceding SOA is controlled (Embodiment 2).

  As shown in FIG. 5, the change in the amount of heat transferred to the DFB-LD1 is suppressed and the fluctuation of the oscillation wavelength of the DFB-LD1 is further suppressed when the front SOA 22 is controlled than when the front SOA 22 is not controlled. can do.

  From the above, according to the second embodiment, fluctuations in the oscillation wavelength of the DFB-LD can be suppressed more than in the first embodiment.

<Embodiment 3>
In the wavelength tunable light source module 9 according to the base technology shown in FIG. 16, the wavelength tunable light source 15 has one output, so that the polarization maintaining coupler 20 has two outputs. However, in such a configuration, a branching loss occurs in the polarization maintaining coupler 20, so that an injection current for an SOA (not shown) provided on the output side of the wavelength tunable light source 15 is increased to compensate for the branching loss. Therefore, although it is necessary to increase the output of the LD output light from the wavelength tunable light source 15, there is a problem that it causes an increase in power consumption and laser oscillation line width.

  The third embodiment of the present invention is characterized in that the wavelength tunable light source has two outputs and is also used as a light source for transmission and reception, and suppresses fluctuations in the oscillation wavelength of the DFB-LD.

  FIG. 6 is a diagram showing an example of the configuration of the wavelength tunable light source according to the third embodiment.

  The wavelength tunable light source according to the third embodiment includes a DFB-LD array 2 composed of 12 DFB-LD1s (semiconductor lasers), an MMI input waveguide 4, and a 12 × 2-MMI 26 (optical multiplexing circuit). The transmission SOA 27 and the reception SOA 30 are provided. A difference from the wavelength tunable light source according to the base technology shown in FIG. 12 is that two-stage SOAs are connected to each MMI output waveguide 5 of the third embodiment for two outputs.

  In order for the tunable light source shown in FIG. 6 to obtain the same amplification factor and optical output as the tunable light source according to the base technology shown in FIG. 12, the length of the front SOA 22 and the length of the rear SOA 23 of each two-stage SOA. Is equal to the length of the SOA 6 (single-stage SOA) shown in FIG. Here, the length of each SOA means the length of the LD output light in the passing direction.

  The 12 × 2-MMI 26 has an input side and an output side, one end of 12 MMI input waveguides 4 is connected to the input side, two MMI output waveguides 5 are connected to the output side, and The LD output light input from the MMI input waveguide 4 is multiplexed, and the combined LD output light is output to the MMI output waveguide 5.

  The twelve DFB-LDs 1 are connected to the other end of each MMI input waveguide 4 and can each perform single mode oscillation at different wavelengths.

  The transmission SOA 27 has a front-stage SOA 28 and a rear-stage SOA 29. The front stage SOA 28 and the rear stage SOA 29 are serially connected to the MMI output waveguide 5 respectively. An SOA connection waveguide 24 is provided between the front stage SOA 28 and the rear stage SOA 29.

  The reception SOA 30 includes a front stage SOA 31 and a rear stage SOA 32. The front stage SOA 31 and the rear stage SOA 32 are serially connected to the MMI output waveguide 5. An SOA connection waveguide 24 is provided between the front stage SOA 31 and the rear stage SOA 32.

  On the exit end side of the two MMI output waveguides 5, waveguide-type SOAs (front-stage SOA 28, rear-stage SOA 29, front-stage SOA 31, rear-stage SOA 32) having different lengths are provided, and the front-stage SOA 28, rear-stage SOA 29, front-stage SOA 31 The electrodes 47 are independently connected to each of the rear SOAs 32 (see FIG. 14).

  The pre-stage SOA 28, the post-stage SOA 29, the pre-stage SOA 31, and the post-stage SOA 32 are formed by removing predetermined portions of the MMI output waveguide 5 and the SOA connection waveguide 24 by etching, and then performing a regrowth technique called butt joint growth to perform the MMI output waveguide. 5 and the SOA connection waveguide 24 are formed so that the cross sections of the front-stage SOA 28, the rear-stage SOA 29, the front-stage SOA 31, and the rear-stage SOA 32 are directly joined. Further, in order to suppress the generation of reflected return light at the output end, a non-reflective coating (not shown) is applied to the output end face. By suppressing the generation of reflected return light, an increase in the laser oscillation line width can be suppressed.

  The configurations of the MMI input waveguide 4, the MMI output waveguide 5, and the SOA connection waveguide 24 are the same as those shown in FIG. 13, and the DFB-LD1, the front-stage SOA 28, the rear-stage SOA 29, the front-stage SOA 31, and the rear-stage SOA 32 Since the configuration is the same as the configuration shown in FIG. 14, the description thereof is omitted here.

  Next, the operation of the wavelength tunable light source will be described.

  When an arbitrary DFB-LD1 is selected and current injection greater than or equal to the threshold current is performed, laser oscillation occurs in the selected DFB-LD1. The LD output light output from the DFB-LD 1 is input to the 12 × 2-MMI 26 multimode region via the MMI input waveguide 4. At this time, if the 12 × 2-MMI 26 is appropriately designed, it is substantially equal to the MMI output waveguide 5 over the entire wavelength band, and is approximately the same as the DFB-LD1 (see FIG. 12) according to the prerequisite technology. LD output light can be coupled at a rate of about 1/12. Therefore, even if the two MMI output waveguides 5 are provided, it is not necessary to increase the drive current as compared with the DFB-LD1 according to the base technology (that is, the drive current of the DFB-LD1 may be similar to that of the base technology).

  When current injection is performed on the transmission SOA 27 and the reception SOA 30, the LD output light traveling through each MMI output waveguide 5 is amplified, and when the current value is increased, the amplification factors in the transmission SOA 27 and the reception SOA 30 also increase. . The amplification factor is expressed by logarithmically expressing the output light intensity based on the input light, but generally exhibits a non-linear behavior. That is, when the input light is weak (low output), the amplification factor is substantially constant. However, when the input light is strong (high output), the amplification factor decreases, so that the maximum output tends to be saturated with the input light intensity. Further, the saturated outputs per unit length of the transmission SOA 27 and the reception SOA 30 are determined by the optical confinement coefficient and the current density of the active layers of the transmission SOA 27 and the reception SOA 30 (corresponding to the InGaAsP active layer 45 in FIG. 14). Therefore, if the optical confinement factors and current densities of the active layers of the transmission SOA 27 and the reception SOA 30 are the same, the maximum outputs of the transmission SOA 27 and the reception SOA 30 are determined by the lengths of the transmission SOA 27 and the reception SOA 30. The

  As shown in FIG. 6, the length of the transmission SOA 27 is longer than the length of the reception SOA 30, and the width of the active layer and the optical confinement factor are the same. Therefore, the maximum output at the same current density is higher in the transmission SOA 27 than in the reception SOA 30. That is, the transmission SOA 27 has a higher amplification factor than the reception SOA 30.

  As described above, in the transmission / reception apparatus for the digital coherent communication system, the reception optical output 13 may be lower than the transmission optical output 7, so the light amplified by the transmission SOA 27 is used as the transmission optical output 7, The light amplified by the reception SOA 30 can be used as the reception optical output 13.

  In other words, if the length of the transmission SOA 27 is the same as the SOA 6 (see FIG. 12) according to the premise technology, the driving current of the DFB-LD 1 and the transmission SOA 27 necessary for obtaining the equivalent transmission optical output 7 The injection current is the same, and the injection current for the reception SOA 30 necessary for obtaining the reception optical output 13 is smaller than that of the base technology. Therefore, when one wavelength tunable light source according to the third embodiment shown in FIG. 6 is used in a transmission / reception device, power consumption is reduced compared to a transmission / reception device (see FIG. 15) based on a premise technique using a separate wavelength tunable light source. It can be reduced to 1/2 or less.

  As described above, according to the third embodiment, it is possible to use the variable wavelength light source as a light source for transmission and reception, and to suppress an increase in laser oscillation line width and power consumption.

  As described in the first embodiment, in the two-stage SOA as shown in FIG. 6, the rear-stage SOA 29 and the rear-stage SOA 32 have a function of turning on / off the LD light output in addition to the function of amplifying the LD output light ( Shutter function). In the third embodiment, a current is always applied to each of the front-stage SOA 28 and the front-stage SOA 31, and the rear-stage SOA 29 and the rear-stage SOA 32 are changed depending on whether the transmission optical output 7 or the reception optical output 13 is output. Whether to apply current to each of them is controlled independently.

  FIG. 7 is a diagram illustrating an example of fluctuations in the oscillation wavelength of the transmission optical output 7 when the reception optical output 13 is switched from OFF to ON. In FIG. 7, the horizontal axis indicates time, and the vertical axis indicates a shift (variation) in the oscillation wavelength of the transmission optical output 7. A broken line indicates the case of the one-stage SOA shown in FIG. 8, and a solid line indicates the case of the two-stage SOA (see FIG. 6).

  In the case of the one-stage SOA and the two-stage SOA, any selected DFB-LD is always ON when the optical output (transmitting optical output 7, receiving optical output 13) is OFF or ON. It is in a state (LD output light is output from the DFB-LD). In the case of a two-stage SOA, the front SOAs (the front SOA 28 and the front SOA 31) in the transmission SOA (transmission SOA 27) and the reception SOA (reception SOA 30) are always ON. When outputting optical output, in the case of a one-stage SOA, the SOA (transmission SOA 33, reception SOA 34) is turned on, and in the case of a two-stage SOA, each succeeding SOA (in the transmission SOA and reception SOA) The rear stage SOA 29 and the rear stage SOA 32) are turned on.

  In addition, regarding the transmission SOA and the reception SOA, the sum of the length of the preceding SOA and the length of the subsequent SOA in the two-stage SOA is assumed to be the same as the SOA length in the one-stage SOA. It is assumed that the transmission optical output 7 and the reception optical output 13 are the same in both the case of the one-stage SOA and the case of the two-stage SOA.

  As shown in FIG. 7, in the case of a two-stage SOA, fluctuations in the oscillation wavelength are suppressed compared to the case of a single-stage SOA.

  Even if the oscillation wavelength is detected by a wavelength monitor and the temperature is controlled by a Peltier element or the like, the oscillation wavelength is fed back by switching the optical output ON / OFF. Since the temperature of the determined DFB-LD varies, the oscillation wavelength also varies. However, the change in the amount of heat is reduced by switching the light output ON / OFF only in each subsequent stage SOA of the two-stage SOA, and the temperature fluctuation of the DFB-LD (that is, the fluctuation of the oscillation wavelength of the DFB-LD, FIG. 7). In the example, it is possible to suppress fluctuations in the oscillation wavelength of the transmission optical output.

  FIG. 7 shows the fluctuation of the oscillation wavelength of the transmission light output when the reception light output is switched from OFF to ON. However, even when the reception light output is switched from ON to OFF, a two-stage type is shown. In the SOA, the same effect as described above can be obtained. Further, even when the transmission optical output is switched from OFF to ON or from ON to OFF, the two-stage SOA can suppress fluctuations in the oscillation wavelength of the reception optical output as described above.

  In the two-stage SOA, when the sum of the length of the front-stage SOA and the length of the rear-stage SOA is the same as the length of the single-stage SOA, the length of the front-stage SOA is longer and the length of the rear-stage SOA is shorter. Variations in the amount of heat (oscillation wavelength variation) when switching on / off the optical output can be suppressed, and the start-up time can be shortened. However, if the length of the post-stage SOA is made too short, the optical output cannot be completely blocked when the optical output is turned off (when the post-stage SOA is turned off). Therefore, it is necessary to adjust the length of the subsequent SOA as necessary (according to the required amount of cutoff).

  In the above description, ON / OFF of the transmission optical output 7 or the reception optical output 13 is switched only by the post-stage SOA 29 and the post-stage SOA 32. However, the oscillation wavelength of the DFB-LD 1 accompanying the change in the amount of heat generated from the post-stage SOA 29 and the post-stage SOA 32 Fluctuations remain. As a countermeasure against such a problem, as described in the second embodiment, by controlling the front-stage SOA 28 and the front-stage SOA 31 as well, it is possible to further suppress fluctuations in the oscillation wavelength of the DFB-LD 1. The configuration of the variable wavelength light source is the same as that shown in FIG.

  Specifically, for the transmission SOA 27 and the reception SOA 30, as shown in FIG. 3, when the optical output (the transmission optical output 7 and the reception optical output 13) is OFF, the preceding SOA (the preceding SOA 28, the preceding SOA). Only SOA 31) is ON. As shown in FIG. 4, when the optical output is ON, the front-stage SOA and the rear-stage SOA (the rear-stage SOA 29, the rear-stage SOA 32) are turned on. While the optical output is in the ON state, the current (that is, electric power) applied to the preceding stage SOA is slightly reduced as compared with the case where the optical output is OFF, and this state is maintained.

  Note that the total power applied to the front-stage SOA and the rear-stage SOA when the optical output is OFF is substantially the same as the total power applied to the front-stage SOA and the rear-stage SOA when the optical output is ON. It is set.

  FIG. 9 is a diagram illustrating an example of fluctuations in the oscillation wavelength of the transmission optical output 7 when the reception optical output 13 is switched from OFF to ON. In FIG. 9, the horizontal axis indicates time, and the vertical axis indicates the shift (variation) in the oscillation wavelength of the transmission optical output 7. A broken line indicates a case where the preceding stage SOA (the preceding stage SOA 28, the preceding stage SOA 31) is not controlled, and a solid line indicates a case where the preceding stage SOA is controlled.

  As shown in FIG. 9, the change in the amount of heat transferred to the DFB-LD 1 is suppressed and the fluctuation of the oscillation wavelength of the DFB-LD 1 is further suppressed when the front stage SOA is controlled than when the front stage SOA is not controlled. can do. That is, fluctuations in the oscillation wavelength of the transmission optical output 7 can be suppressed.

  Although FIG. 9 shows the fluctuation of the oscillation wavelength of the transmission optical output 7, the same effect can be obtained for the reception optical output 13.

  From the above, according to the third embodiment, it is possible to use the wavelength tunable light source as two outputs as a light source for transmission and reception, and to suppress an increase in laser oscillation line width and power consumption. In addition, fluctuations in the oscillation wavelength of the DFB-LD can be suppressed.

<Embodiment 4>
FIG. 10 is a diagram showing an example of the configuration of the wavelength tunable light source module 35 according to Embodiment 4 of the present invention.

  The fourth embodiment is characterized in that the variable wavelength light source (see FIG. 6) according to the third embodiment is used as the variable wavelength light source 36 of the variable wavelength light source module 35. Other configurations and operations of the wavelength tunable light source module 35 are the same as those of the wavelength tunable light source module 9 according to the base technology of FIG.

  As shown in FIG. 10, the two output ports of the wavelength tunable light source module 35 are imaged by being separated into two optical fibers 17 by the coupling optical system 16, and the light output from each optical fiber 17 is used for transmission. It can be used as the optical output 7 and the receiving optical output 13. That is, each of the plurality of output lights output from the wavelength variable light source 36 is taken out separately.

  From the above, according to the fourth embodiment, the polarization maintaining coupler 20 provided on the output side of the wavelength tunable light source module 9 according to the base technology of FIG. As a result, an increase in the laser oscillation line width can be suppressed. Further, since the wavelength variable light source 36 uses a two-stage SOA, the optical output 7 for transmission is switched by switching ON / OFF of the optical output (transmitting optical output 7 and receiving optical output 13) only in the subsequent SOA. To suppress fluctuations in the oscillation wavelength of the reception optical output 13 when the ON / OFF is switched, and fluctuations in the oscillation wavelength of the transmission optical output 7 when the ON / OFF of the reception optical output 13 is switched. it can.

<Embodiment 5>
FIG. 11 is a diagram illustrating an example of the configuration of the transmission / reception device 37 according to the fifth embodiment of the present invention.

  The fifth embodiment is characterized in that the transmitting / receiving device 37 includes the variable wavelength light source module 35 according to the fourth embodiment.

  The optical output 7 for transmission output from the wavelength variable light source module 35 is modulated by the modulator module 10 and then output to the outside as the transmission signal 11.

  The received signal 12 received from the outside is input to the receiving module 14 together with the receiving light output 13 output from the wavelength variable light source module 35, and is restored after the signal processing.

  Thus, the wavelength variable light source module 35 is also used as a light source module for transmission and reception.

  From the above, according to the fifth embodiment, the wavelength tunable light source module 35 can be used as a light source module for transmission / reception. Therefore, the transmission / reception apparatus 8 (2) using the two wavelength tunable light source modules 9 according to the base technology ( The power consumption can be reduced as compared with FIG. Further, since the wavelength tunable light source used in the wavelength tunable light source module 35 is a two-stage SOA, by switching ON / OFF of the optical output (transmitting optical output 7, receiving optical output 13) only in the subsequent SOA, Variation in oscillation wavelength of reception optical output 13 when ON / OFF of transmission optical output 7 is switched, and variation of oscillation wavelength of transmission optical output 7 when ON / OFF of reception optical output 13 is switched Can be suppressed.

  It should be noted that the present invention can be freely combined with each other within the scope of the invention, and each embodiment can be appropriately modified or omitted.

  DESCRIPTION OF SYMBOLS 1 DFB-LD, 2 DFB-LD array, 3 Nx1-MMI, 4 MMI input waveguide, 5 MMI output waveguide, 6 SOA, 7 Optical output for transmission, 8 Transmission / reception apparatus, 9 Wavelength variable light source module, 10 Modulator module, 11 transmission signal, 12 reception signal, 13 reception optical output, 14 reception module, 15 wavelength variable light source, 16 coupling optical system, 17 optical fiber, 18 beam splitter, 19 monitor, 20 polarization maintaining coupler, 21 12 × 1-MMI, 22 front-stage SOA, 23 rear-stage SOA, 24 SOA connection waveguide, 25 optical output, 26 12 × 2-MMI, 27 transmission SOA, 28 front-stage SOA, 29 rear-stage SOA, 30 reception SOA, 31 Front-stage SOA, 32 Rear-stage SOA, 33 Transmitting SOA, 34 Receiving SOA, 35 Wavelength variable light source module 36, 36 wavelength tunable light source, 37 transceiver, 40 InP substrate, 41 InP lower clad layer, 42 InGaAsP waveguide layer, 43 InP upper clad layer, 44 InP current blocking layer, 45 InGaAsP active layer, 46 InGaAsP contact layer, 47 electrodes.

Claims (5)

One end of a plurality of input waveguides is connected to the input side, and at least one or more output waveguides are connected to the output side, and input from each of the input waveguides An optical multiplexing circuit for combining light and outputting the combined light to the output waveguide;
A plurality of semiconductor lasers connected to the other end of each of the input waveguides, each capable of single mode oscillation at different wavelengths;
A pre-stage optical amplifier connected to the optical waveguide side of the output waveguide;
A post-stage optical amplifier connected to the post-stage side of the pre-stage optical amplifier of the output waveguide;
With
The light output from the output waveguide is controlled to be turned ON / OFF by the post-stage optical amplifier ,
The power consumed by the front-stage optical amplifier and the rear-stage optical amplifier when the output of the light output from the output waveguide is ON, and the front-stage optical amplifier when the output of the light output from the output waveguide is OFF and the like equal to the power consumed by the rear-stage optical amplifier, characterized that you control the front-stage optical amplifier and the rear-stage optical amplifier, the wavelength tunable light source.   The wavelength tunable light source according to claim 1, wherein the number of the output waveguides is one.   The wavelength tunable light source according to claim 1, wherein the number of the output waveguides is two. The current applied at the said ON for front-stage optical amplifier, characterized in that it is controlled to be smaller than the current applied the OFF at the wavelength tunable light source according to claim 1. A wavelength tunable light source according to any one of claims 1 to 4,
The wavelength tunable light source module, wherein output light output from the wavelength tunable light source can be taken out.

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