JP4141715B2 - Wavelength control device, wavelength control method, and wavelength tunable semiconductor laser device for wavelength tunable semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength control device and a wavelength control method for a wavelength tunable semiconductor laser that controls the oscillation wavelength of a wavelength tunable semiconductor laser used as an optical communication device applied to wavelength division multiplex transmission (WDM) and the like. The present invention relates to a wavelength tunable semiconductor laser device on which the device is mounted.
[0002]
[Prior art]
In an optical communication system using an optical fiber, a dense wavelength division multiplexing (DWDM) system has been performed. This DWDM system is a system in which a plurality of different wavelengths are multiplexed and transmitted on a single optical fiber, and it is necessary to stabilize the wavelength of light with high accuracy.
[0003]
As a light source used in the DWDM system, a distributed feedback (DFB) laser is generally used. In this DFB laser, a diffraction grating that selectively reflects only one wavelength is formed in the optical amplification region. For this reason, when a DFB laser is used, the oscillation mode is stabilized and a single wavelength semiconductor laser can be realized. Normally, in an optical device used for a WDM system, one light source is used for one channel (wavelength). However, since a DFB laser has a small wavelength variable region, a spare light source for troubleshooting is also provided with one light source for one channel. Since a DFB laser is required, there is a problem that the system becomes expensive.
[0004]
In order to overcome the above problem, it is necessary to use a laser having a large wavelength variable region as a light source. In a DBR (Distributed Bragg Reflector) semiconductor laser, a wavelength-dependent diffraction grating is arranged on both sides of an optical amplification region, and only a specific wavelength is selectively reflected and amplified in the optical amplification region. Oscillation light having a peak wavelength is generated. At this time, the oscillation wavelength can be changed by about several tens of nanometers by changing the injection current to the diffraction grating portions on both sides. However, in a semiconductor laser having a DBR structure in which the reflection peak interval is relatively narrow, due to mode hopping in which the wavelength at which the reflection peaks of the diffraction gratings on both sides coincide with each other jumps to each other, competition between adjacent oscillation longitudinal modes, There is a problem that the oscillation mode tends to become unstable.
[0005]
[Problems to be solved by the invention]
For this reason, in the past, a method of adjusting while checking the oscillation mode using a spectrum analyzer is often used. However, since the spectrum analyzer is used, the wavelength adjustment control becomes large and the size of the apparatus increases. There is.
[0006]
As another prior art, a two-dimensional data table is created with the injection currents I1 and I2 to the diffraction grating portions on both sides as variables, and data relating to oscillation characteristics for each combination of the injection currents I1 and I2 (single It is known to register a single mode oscillation wavelength and data indicating unstable oscillation) and perform wavelength control using this registration data.
[0007]
However, in this prior art, since the data relating to the oscillation characteristics is registered for every combination of the injection currents I1 and I2, the registration data in the data table increases and a huge memory capacity is required. As a result, the apparatus cost increases, and it becomes difficult to reduce the size to such an extent that it can be housed in the optical transmission module.
[0008]
The present invention has been made in view of the above, and a wavelength control device, a wavelength control method, and a wavelength tunable device for a wavelength tunable semiconductor laser capable of performing highly accurate and stable wavelength control using a simplified database with little stored data. An object is to obtain a semiconductor laser device.
[0009]
[Means for Solving the Problems]
  A wavelength control device for a wavelength tunable semiconductor laser according to the present invention includes a first and second light reflectors having a plurality of reflection peaks, and an active layer region disposed between the first and second light reflectors. In the wavelength control device for a wavelength tunable semiconductor laser that controls the oscillation wavelength of the wavelength tunable semiconductor laser having the above, the relationship between the injection current to the first optical reflector and the injection current to the second optical reflector and the oscillation wavelength A database that stores a function that passes through a plurality of regions that stably oscillate from or information for defining the function, and the first and second light reflectors that correspond to a target wavelength based on the stored data of the database And a power supply controller that acquires an injection current value to the first and second optical reflectors using the acquired injection current value.In the above database, when the injection current to the first light reflector is If and the injection current to the second light reflector is Ir, the following function or constants a for defining the following function: It has one set to multiple sets of data for b, c, d.
  Ir = a × If + b
  If = (c × λ + d) ² (Λ: wavelength, a, b, c, d: constant)
[0011]
The semiconductor laser may have a DBR structure using a non-uniform grating in which the first and second light reflectors have different grating spacing periods.
[0012]
The wavelength tunable semiconductor laser further includes an oscillation mode monitor that has a phase control region between the active layer region and the first or second optical reflector and monitors an oscillation mode, and the oscillation mode monitor And an oscillation mode control circuit for controlling an injection current to the phase control region based on the output of.
[0013]
The oscillation mode monitor may determine whether the oscillation mode is single longitudinal mode oscillation or multimode oscillation based on an alternating current component of oscillation light.
[0014]
Further, a wavelength monitor for monitoring the oscillation wavelength may be further provided, and a wavelength control circuit for controlling the element temperature or the injection current of the wavelength tunable semiconductor laser based on the output of the wavelength monitor may be further provided.
[0015]
The wavelength monitor may include a wavelength filter whose transmittance changes according to the wavelength of the input oscillation light, and a photodetector that receives the transmitted light of the wavelength filter.
[0016]
The wavelength monitor includes a narrow-band wavelength filter whose transmittance changes according to the wavelength of the input oscillation light, a first photodetector that receives the transmitted light of the narrow-band wavelength filter, and the input oscillation light. And a second wavelength detector that receives light transmitted through the broadband wavelength filter.
[0017]
The wavelength discrimination region of the narrow-band wavelength filter may correspond to an ITU grid, and the wavelength discrimination region of the wide-band wavelength filter may be larger than the wavelength variable range of the semiconductor laser.
[0018]
The wavelength filter may be any of a Fabry-Perot etalon, a birefringent filter, a multilayer filter, and a fiber grating.
[0019]
And a light intensity monitor that detects the light intensity of the oscillation light, and further includes a light intensity control circuit that controls an injection current into the active layer region so that a detection output of the light intensity monitor is constant. Also good.
[0020]
  A wavelength control method for a wavelength tunable semiconductor laser according to the present invention includes a first and second light reflectors having a plurality of reflection peaks, an active layer region disposed between the first and second light reflectors, and In a wavelength control method of a wavelength tunable semiconductor laser for controlling an oscillation wavelength of a wavelength tunable semiconductor laser having a phase control region, an injection current to the first optical reflector and an injection current to the second optical reflector and oscillation The first and second light reflectors corresponding to the target wavelength based on data stored in a database that stores a function passing through a plurality of regions that stably oscillate from a relationship with the wavelength or information for defining the function A light reflector control step of obtaining an injection current value into the first and second light reflectors using the obtained injection current value;The above database has the following functions or constants a, b, and a constant for defining the following functions when the injection current to the first light reflector is If and the injection current to the second light reflector is Ir. It has one set to multiple sets of data for c and d.
  Ir = a × If + b
  If = (c × λ + d) ² (Λ: wavelength, a, b, c, d: constant)
[0021]
Further, an oscillation mode control step that is executed after the optical reflector control step and controls an injection current to the phase injection region based on the detected oscillation mode may be further provided.
[0022]
Further, it may further include a wavelength control step that is executed after the optical reflector control step and controls an element temperature or an injection current of the wavelength tunable semiconductor laser based on the detected oscillation wavelength.
[0023]
Further, a light intensity control step for controlling an injection current to the active layer region so that the detected light intensity is constant may be further provided.
[0025]
  A wavelength tunable semiconductor laser device according to the present invention includes a first and second light reflectors having a plurality of reflection peaks, and an active layer region and a phase control region disposed between the first and second light reflectors. And a function that passes through one or more regions that stably oscillate from the relationship between the injection current to the first optical reflector and the injection current to the second optical reflector and the oscillation wavelength, or A database storing information for defining the function, an oscillation mode monitor for monitoring the oscillation mode, and injection to the first and second optical reflectors corresponding to the target wavelength based on the stored data of the database A power supply controller that acquires a current value and controls the injection current to the first and second optical reflectors using the acquired injection current value, and the phase control based on the output of the oscillation mode monitor. Bei an oscillation mode control circuit for controlling the current injected into the regionIn the above database, when the injection current to the first light reflector is If and the injection current to the second light reflector is Ir, the following function or constants a for defining the following function: It has one set to multiple sets of data for b, c, d.
  Ir = a × If + b
  If = (c × λ + d) ² (Λ: wavelength, a, b, c, d: constant)
[0027]
Further, an optical branching device for branching the laser beam on the optical axis of the front light output of the wavelength tunable semiconductor laser and inputting one of the branched outputs to the oscillation mode monitor may be further provided.
[0028]
The oscillation mode monitor may be arranged to receive part or all of the back light of the wavelength tunable semiconductor laser.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Exemplary embodiments of a wavelength control device, a wavelength control method, and a wavelength variable semiconductor laser device according to the present invention will be explained below in detail with reference to the accompanying drawings.
[0030]
FIG. 1 is a block diagram showing the configuration of a wavelength tunable semiconductor laser device to which the present invention is applied. As shown in FIG. 1, the wavelength tunable semiconductor laser device includes a DBR semiconductor laser 1 and a heat sink 2 disposed adjacent to the chip of the semiconductor laser 1 so as to compensate the temperature of the chip of the semiconductor laser 1 at a constant temperature. And Peltier element 3, beam splitters 4, 5, and 6 as optical branching means, wavelength monitor 7 for detecting the wavelength of the laser beam, oscillation mode monitor 8 for detecting the oscillation mode state, and detecting the laser beam intensity. A light intensity monitor 9, a temperature monitor 10 for monitoring the temperature of the chip of the semiconductor laser 1, and a semiconductor laser driving device 20 for controlling the semiconductor laser 1 are provided.
[0031]
The semiconductor laser driving device 20 includes a wavelength control circuit 21 that controls the wavelength of the laser light based on the output of the wavelength monitor 7, an oscillation mode control circuit 22 that controls the oscillation mode of the laser light based on the output of the oscillation mode monitor 8, A light intensity control circuit 23 for controlling the light intensity to be constant based on the output of the light intensity monitor 9 and a temperature control circuit for controlling the temperature of the chip of the semiconductor laser 1 to be constant based on the output of the temperature monitor 10. 24, a database (DB) 25 storing parameters necessary for controlling the semiconductor laser 1 (parameters for obtaining an injection current into the DBR region), a laser element for injecting current into the semiconductor laser 1 And a power supply controller 26 for controlling the temperature.
[0032]
The output light from the semiconductor laser 1 is branched by the beam splitter 4 and a part thereof is extracted as monitor light A1. The monitor light A1 is branched by the beam splitter 5, and a part thereof is input to the wavelength monitor 7 as the branched light A2. The remaining light is incident on the beam splitter 6 and branched. A part of the light enters the oscillation mode monitor 8 as the branched light A3, and the remaining light enters the light intensity monitor 9 as the branched light A4.
[0033]
A wavelength monitor signal that is an output of the wavelength monitor 7 is input to the wavelength control circuit 21. An oscillation mode monitor signal that is an output of the oscillation mode monitor 8 is input to the oscillation mode control circuit 22. The light intensity monitor signal that is the output of the light intensity monitor 9 is input to the light intensity control circuit 23. Further, a temperature monitor signal that is an output of the temperature monitor 10 is input to the temperature control circuit 24.
[0034]
In the semiconductor laser driving device 20, data from the database 25 and signals from the wavelength control circuit 21, the oscillation mode control circuit 22, the light intensity control circuit 23, and the temperature control circuit 24 are input to the power supply controller 26. The power controller 26 controls the power supply based on these input signals, thereby controlling the injection current to the semiconductor laser 1 and the drive current of the Peltier element 3.
[0035]
Next, details of the semiconductor laser 1, the wavelength monitor 7, the oscillation mode monitor 8, and the light intensity monitor 9 will be described.
[0036]
First, the semiconductor laser 1 as a light source will be described. In this case, as the semiconductor laser 1, for example, a superstructure grating DBR semiconductor laser (hereinafter abbreviated as SSG DBR-LD) is used. FIG. 2 shows a typical structure of SSG DBR-LD.
[0037]
In FIG. 2, the front SSG DBR region 31 as one of the light reflectors constituting the resonator has a structure in which non-uniform diffraction gratings (non-uniform gratings) with varying lattice intervals are arranged in multiple stages in the optical axis direction. Yes, a reflection spectrum having a plurality of reflection peaks can be realized. The rear SSG DBR region 34 as the other light reflector has a structure in which diffraction gratings with different periodic intervals from the front SSG DBR region 31 are arranged in multiple stages, and the front SSG DBR region 31 has a plurality of reflection peak periods different from each other. A reflection spectrum having a reflection peak can be realized. That is, the DBR regions 31 and 34 are obtained by arranging a group of diffraction gratings (chirped gratings) whose grating intervals (periods) continuously change as one unit, and arranging these diffraction grating groups at the same intervals an integer number of times. Basically, the front SSG DBR region 31 and the rear SSG DBR region 34 have different reflection peaks between the front SSG DBR region 31 and the rear SSG DBR region 34 by changing the period of the arrangement of the chirped gratings. The period of is different. Note that changing the period of the chirped grating array is equivalent to changing the length of the chirped grating alone, and in the chirped grating (one unit) to keep the reflection characteristics of the single chirped grating. Changes in the period may also be changed.
[0038]
Between the front SSG DBR region 31 and the rear SSG DBR region 34, an active layer region 32 as a gain region and a phase control region 33 having no gain effect are arranged. Each of the regions 31 to 34 has a structure capable of injecting current independently. The injection current to the active layer region 32 is Ia, the injection current to the phase control region 33 is Ip, the injection current to the front SSG DBR region 31 is If, and the injection current to the rear SSG DBR region 34 is Ir. .
[0039]
The operation of the semiconductor laser 1 will be described. When a driving current Ia equal to or higher than the threshold value in the active layer region 32 is injected, light that resonates between the front SSG DBR region 31 and the rear SSG DBR region 34 is amplified in the active layer region 32, and from the front SSG DBR region 31. The emitted light is extracted.
[0040]
The oscillation wavelength at this time is determined by gain characteristics and loss characteristics as shown in FIG. In FIG. 3, the gain characteristic 40, the reflection characteristic (thick line) 41 of the front SSG DBR region 31, the reflection characteristic (dotted line) 42 of the rear SSG DBR region 34, the front SSG DBR region 31 and the rear SSG DBR region 34, A resonance longitudinal mode 43 (thin line) determined by the resonator length between the two is shown. In each reflection characteristic and resonance longitudinal mode, the reflectance is maximum at the peak. The period of the reflection peak 41 in the front SSG DBR region 31 and the period 42 (wavelength interval) of the reflection peak in the rear SSG DBR region 34 are slightly different as described above.
[0041]
Within the gain band 40, the wavelength at which the reflection peak of the front SSG DBR region 31 and the reflection peak of the rear SSG DBR region 34 coincide is called an SSG mode, and oscillates at a wavelength at which the SSG mode and the resonance longitudinal mode coincide. . In the case of FIG. 3, the SSG mode and the resonance longitudinal mode coincide with each other at the wavelength λ0 near the center, and oscillation occurs at this wavelength λ0.
[0042]
The gain band and the SSG mode are adjusted by changing the currents Ia, If, Ir to be injected into the respective elements and the element temperature Tld of the semiconductor laser 1, and the resonant longitudinal mode is changed by changing the injection current Ip to the phase control region 33. To adjust the wavelength.
[0043]
FIG. 4 shows the distribution of the oscillation wavelength when If and Ir are changed under the condition that the injection current Ia into the active layer region 32 is constant. FIG. 5 shows the wavelength boundaries of the wavelength distribution in FIG. 4 by solid lines and broken lines, because FIG. 4 is divided into wavelengths by color and is difficult to see for convenience of explanation in a monochrome state. is there. 5, the wavelength boundary in the direction extending from the upper left to the lower right, which is particularly difficult to see in FIG. 4, is indicated by a broken line.
[0044]
FIG. 4 shows that the oscillation wavelength becomes longer as the color becomes darker. In addition, in FIG. 4, although it is difficult to visually recognize, in each of a plurality of regions surrounded by a solid line extending from the lower left to the upper right on the paper surface shown in FIG. It changes to the wavelength side. The boundary of this continuous change corresponds to the broken line shown in FIG.
[0045]
On the other hand, when either If or Ir is made constant and the other is continuously changed, there is a point where the wavelength changes abruptly (shown as a small rectangular block K in FIG. 4). This is because the reflection characteristics of the front SSG DBR region 31 and the rear SSG DBR region 34 shown in FIG. 3 change with respect to the wavelength. That is, when only one of the reflection characteristics 41 and 42 of the front SSG DBR region 31 and the rear SSG DBR region 34 is changed, the SSG mode shifts to the adjacent reflection peak due to the vernier effect, and the wavelength jumps greatly. Caused by mode hopping.
[0046]
FIG. 6A shows, as an example, an oscillation spectrum measured under a current condition in which the wavelength changes rapidly (point A in FIGS. 4 and 5: If = 13.8 mA, Ir = 17 mA). FIG. 6B shows an oscillation spectrum measured under other current conditions (point B in FIGS. 4 and 5: If = 18 mA, Ir = 17 mA). Note that point B is selected in the vicinity of the center of one scale-like region surrounded by a solid line and a broken line in FIG.
[0047]
As shown in FIG. 6A, oscillation occurs at two different wavelengths at point A, resulting in multimode oscillation. Note that the oscillations at these two wavelengths are not always oscillating, and one of them oscillates irregularly on the time axis. This is due to the competition between the above-mentioned adjacent SSG modes, and therefore oscillation is unstable. On the other hand, as shown in FIG. 6B, single longitudinal mode oscillation is obtained at point B where the vicinity of the center of the scale-like region is selected.
[0048]
On the other hand, unstable oscillation may occur in addition to SSG mode competition. That is, in FIGS. 4 and 5, even when If and Ir are continuously changed from the lower left to the upper right, the wavelength jumps a little. This indicates that the oscillation longitudinal mode has shifted to an adjacent mode, and in this case, oscillation becomes unstable.
[0049]
Summarizing these two types of mode jumps, the central portion of the plurality of scale-like regions described above is a region where a single longitudinal mode oscillates stably, and its boundary portion (regions in the vicinity of the solid and broken lines in FIG. 5). ) Shows that the oscillation becomes unstable. The present invention has been made based on such considerations. A predetermined function capable of extracting only the conditions for stable oscillation is set in the database 25, and a constant value for defining this function is associated with the oscillation wavelength. And remember. Details thereof will be described later.
[0050]
Next, the wavelength monitor 7 will be described. The wavelength monitor 7 is a device that discriminates the wavelength of input light and outputs a wavelength monitor signal that changes in accordance with the wavelength, and includes, for example, a wavelength filter and a photodetector. The wavelength filter has a characteristic of changing the transmittance according to the input light wavelength. As the wavelength filter, for example, a Fabry-Perot etalon, a birefringence filter, a thin film filter, a fiber grating, or the like is used. Wavelength information is converted into intensity information by the wavelength filter using the characteristic of changing the transmittance according to the input light wavelength of the wavelength filter. An optical signal having an intensity corresponding to the wavelength can be obtained by converting the optical signal from the wavelength filter into an electrical signal by the photodetector.
[0051]
Now, let the oscillation target wavelength of the semiconductor laser 1 to be stabilized be λc. If the oscillation wavelength shifts by Δλ, the intensity of the wavelength monitor signal increases or decreases accordingly. By monitoring the increase / decrease, the wavelength variation can be detected. Therefore, the wavelength control circuit 21 adjusts one of the injection currents Ip, Ia, If, Ir, or the element temperature Tld so that the wavelength monitor signal has the intensity at the target wavelength λc, thereby changing the wavelength to the target wavelength λc. Can be stabilized.
[0052]
Next, the oscillation mode monitor 8 will be described. The oscillation mode monitor 8 determines whether single longitudinal mode oscillation or multimode oscillation is performed according to the oscillation mode state, and outputs an oscillation mode monitor signal indicating the determination result. As the oscillation mode monitor 8, for example, a photodetector that can detect only an AC component excluding a DC component of input light is used. When the oscillation mode becomes unstable due to mode competition or the like, the intensity noise increases. Therefore, the output from the photodetector tends to increase when the multimode oscillation occurs, and decrease when the single longitudinal mode oscillation occurs. A threshold is set for the output signal of the photodetector, and if it is equal to or greater than the threshold, it is determined that multimode oscillation is performed, and if it is equal to or less than the threshold, single longitudinal mode oscillation is determined. The oscillation mode control circuit 22 stabilizes the oscillation mode by controlling the injection current Ip to the semiconductor laser 1 based on the oscillation mode monitor signal.
[0053]
Next, the light intensity monitor 9 will be described. The light intensity monitor 9 outputs a signal corresponding to the change in the intensity of the laser light as a light intensity monitor signal. An example of the configuration is a photodetector. The photodetector has a characteristic that the output signal increases when the input light intensity increases, and the output signal decreases when the input light intensity decreases. Therefore, by monitoring this signal, it is possible to detect a change in laser beam intensity. The light intensity control circuit 23 can stabilize the laser light intensity to a desired value by controlling the injection current Ia to the active layer region 32 so that the light intensity monitor signal is constant.
[0054]
The semiconductor laser driving device 20 controls the injection current (Ia, If, Ir, Ip) to the semiconductor laser 1 based on the wavelength monitor signal, the oscillation mode monitor signal, the laser light intensity monitor signal, and the temperature monitor signal, The element temperature (Tld) is controlled by controlling the Peltier element 3. The operator automatically adjusts to the target wavelength and the target intensity and oscillates in the single longitudinal mode only by inputting the target wavelength and the laser light intensity to be stabilized in the semiconductor laser driving device.
[0055]
Next, wavelength tuning operation will be described. It is assumed that the laser light intensity is stabilized at a target intensity set in advance by the light intensity control circuit 23 based on the laser light intensity monitor signal. FIG. 7 is a flowchart showing the wavelength tuning operation of the wavelength tunable semiconductor laser device shown in FIG. 1, and the operation will be described according to this flowchart.
[0056]
First, the database 25 forming the main part of the present invention will be described. When setting If and Ir, it is necessary to avoid unstable mode oscillation. However, if all current maps as shown in FIG. 4 are recorded as a database, a single longitudinal mode oscillation region in the figure is recorded. It can be set, but the data capacity becomes enormous.
[0057]
Therefore, as shown in FIG. 8, conditions are set such that the lines L1 to L4 are set so as to be positioned substantially in the middle of a plurality of regions surrounded by a solid line extending from the lower left to the upper right, and the lines L1 to L4 are placed By introducing the relational expression of If, Ir, and wavelength below, the transition to the transition region of the SSG mode is avoided and the amount of stored data is greatly reduced. In addition, wavelength control that is strong against aging of the laser element is realized by selecting the vicinity of the center as far as possible from the transition region as much as possible.
[0058]
First, as shown in FIG. 8, the relationship between If and Ir that obtains a stable oscillation mode away from the transition region of the SSG mode (the wavelength boundary region indicated by the solid line in FIG. 8) is obtained.
[0059]
This work corresponds to selecting one of the SSG modes, which is the reflection mode of the front DBR region 31 and the rear DBR region 34 (the reflection characteristic of the front DBR region 31 in FIG. 3 (thick line)). 41 and the reflection characteristic (dotted line) 42 of the rear DBR region 34 overlap). The relationship is a straight line, and is expressed by the equation Ir = a × If + b (a and b are constants). For example, the relationship between If and Ir in the line L1 is Ir = 0.8 × If + 2.0, and Ir that satisfies this expression may be set when a certain If is given. The same applies to the lines L2 to L4.
[0060]
Next, FIG. 9 shows the relationship between If and wavelength on each of the lines L1 to L4. The relationship between the wavelength λ and If on each line L1 to L4 is If = (c × λ + d)²(C and d are constants).
[0061]
Therefore, if you want to set the target wavelength λc, first select on which line the target wavelength is, and on the selected line,
Ir = a × If + b (1)
If = (c × λ + d) ^ 2 Equation (2)
If and Ir satisfying the above are obtained, and the obtained If and Ir are applied. In the database 25, for each of the lines L1 to L4,
(1) Selectable wavelength range of each line L1 to L4
(2) Constants a and b in equation (1)
(3) Constants c and d in equation (2)
It is only necessary to memorize. Note that the relationship between Ir and wavelength may be used in the above formula (2). Further, the functions themselves of the expressions (1) and (2) in which the constants a, b, c, and d are defined may be stored in the database 25 corresponding to the oscillation wavelength.
[0062]
However, only such setting of If and Ir makes it possible to control the wavelength away from the transition region indicated by the solid line in FIG. 8, but a transition to the transition region indicated by the broken line in FIG. 8 is still assumed. For this reason, the above-described setting of If and Ir alone does not stabilize the target wavelength with high accuracy, but stabilizes the oscillation longitudinal mode by Ip control described later and wavelength stabilization by wavelength monitoring (element temperature Tld control, etc.). ) Is required.
[0063]
Next, the wavelength tuning procedure using the database 25 will be described with reference to the flowcharts shown in FIGS.
[0064]
First, the operator inputs the wavelength of the target wavelength λc to be stabilized into the semiconductor laser driving device 20 (step 100). Specifically, an operator inputs to the power supply controller 26 via an input device (not shown).
[0065]
Next, the power supply controller 26 in the semiconductor laser driving apparatus 20 refers to the database 25 to obtain If and Ir, and the obtained If and Ir are stored in the front SSG DBR region 31 and the rear SSG DBR region 34 of the semiconductor laser 1. The oscillation wavelength is made to reach the vicinity of the target wavelength λc (step 110).
[0066]
The DBR region application current process performed in step 110 is shown in the flowchart of FIG. First, when the target set wavelength λc is input (step 200), the line determination process in steps 210, 230, 250, and 270 is used to determine which line the target wavelength λc is on with reference to the database 25. .
[0067]
If the target wavelength λc is in the range of λ1 ≦ λc ≦ λ2 (step 210), If and Ir are obtained using the relational expressions (1) and (2) for one line (step 220). That is, constants a, b, c, and d set in the selected line are read from the database 25, and If and Ir are determined using the equations (1) and (2).
[0068]
If the target wavelength λc is in the range of λ2 <λc ≦ λ3 (step 230), If and Ir are obtained using the relational expressions (1) and (2) for another line (step 240). If the target wavelength λc is in the range of λ3 <λc ≦ λ4 (step 250), If and Ir are obtained using the relational expressions (1) and (2) for another line (step 260). If the target wavelength λc is in the range of λ4 <λc ≦ λ5 (step 270), If and Ir are obtained using the relational expressions (1) and (2) for another line (step 280).
[0069]
If it does not correspond to the wavelength on all the lines, it indicates that it is outside the oscillation wavelength of the semiconductor laser 1 and displays an error (step 290).
[0070]
In this case, in step 210, the selection range of the target wavelength λc is set to λ1 ≦ λc ≦ λ2, in step 230, the selection range of the target wavelength is set to λ2 <λc ≦ λ3, and in step 250, the selection range of the target wavelength is set to λ3 < In step 270, the target wavelength selection range is set to λ4 <λc ≦ λ5 so that the target wavelength selection ranges do not overlap in each step, and each step 210, 230, 250, 270 is performed. When the condition is satisfied, if one line is selected and If and Ir are obtained by using the relational expressions (1) and (2) for the selected one line, the target wavelength λc is the semiconductor laser. If it is within one oscillatable wavelength range λ1 ≦ λc ≦ λ5, only one line is selected.
[0071]
If the line with the larger injection current is selected for the only line selection, there is a drawback that the current becomes large, but more stable wavelength control can be achieved. This is because, as can be seen from FIGS. 4 and 5, the larger the current values If and Ir, the larger the scale-shaped region described above, and the points on the line are further away from the transition region.
[0072]
Next, the oscillation mode monitoring process at step 120 in FIG. 7 will be described. The oscillation mode control circuit 22 refers to the signal from the oscillation mode monitor 8 (step 120), and determines whether the mode currently oscillating is single mode or multimode oscillation (step 130). When the oscillation mode is stable, the signal intensity from the oscillation mode monitor 8 is low. Conversely, when the oscillation mode is unstable, the signal intensity is large. Therefore, the oscillation mode control circuit 22 compares the reference value with a preset reference value, and if the signal is equal to or less than the reference value, the procedure proceeds to the next step 150. The oscillation mode is finely adjusted and stabilized by increasing the injection current Ip to the phase control region 33 from 0 until it becomes equal to or lower than the threshold (step 140). Since the optimum If and Ir are selected in the DBR region application current processing in the previous step 110, it is not necessary to consider unstable mode oscillation due to the competition of the SSG mode. Thus, there is no need to adjust If and Ir. By this control, the oscillation mode can be stabilized by shifting the oscillation mode from the transition region on each of the lines L1 to L4.
[0073]
Next, the wavelength control circuit 21 detects the amount of deviation from the target wavelength λc based on the wavelength monitor signal from the wavelength monitor 7 (steps 150 and 160). The wavelength control circuit 21 drives the Peltier element 3 via the power supply controller 26 and adjusts the temperature near the semiconductor laser by the temperature monitor 10 when the amount of wavelength deviation is larger than a predetermined threshold (oscillation wavelength accuracy). By adjusting the element temperature Tld while monitoring, the target wavelength λc is stabilized (step 170). Further, the wavelength control circuit 21 displays that the target wavelength is stabilized when the wavelength shift amount is smaller than the oscillation wavelength accuracy.
[0074]
In this case, in the wavelength control based on the output of the wavelength monitor 7, the wavelength is finely adjusted by controlling only the element temperature Tld. This is because when the wavelength control is performed by controlling any one of the injection currents Ip, Ia, If and Ir in the steps 150 to 170, the Ip, If and Ir adjusted in the previous steps 110 and 140 are obtained. This is because it has been considered that the change may adversely affect the oscillation mode.
[0075]
During the operation of the semiconductor laser device, the procedure from step 120 to step 180 is repeatedly executed.
[0076]
Thus, according to the embodiment, the relational expression Ir = a × If + b that avoids unstable mode oscillation due to SSG mode competition, and the relational expression If = wave at that time If = (c × λ + d)², And using a simplified database in which the constants a, b, c, d of these relational expressions or the relational expressions themselves are stored in correspondence with the oscillation wavelength range, wavelength tuning for obtaining a stable single longitudinal mode oscillation is used. The storage capacity of the database when performing the process is greatly reduced, contributing to downsizing and cost reduction of the apparatus.
[0077]
Further, in the wavelength tuning operation, by using the output of the oscillation mode monitor 8 to control the injection current Ip to the phase control region 33, the transition to the transition region on the line selected by the control of If and Ir is performed. Since this is avoided, transmission in the single longitudinal mode is stabilized with high accuracy. Further, since the element temperature Tld is adjusted using the wavelength monitor 7 in the wavelength tuning operation, the oscillation wavelength can be accurately detected without affecting the other control parameters such as the injection currents Ip, If, Ir to the semiconductor laser 1. There is an effect that can be stabilized.
[0078]
Next, another configuration example of the wavelength monitor 7 used in the laser apparatus of FIG. 1 will be described with reference to FIGS. A wavelength monitor 7 shown in FIG. 11 includes a narrowband wavelength filter 50 whose transmittance changes in accordance with the wavelength of input oscillation light (monitor light), and a photodetector 51 that receives the transmitted light of the narrowband wavelength filter 50. A broadband wavelength filter 52 whose transmittance changes according to the wavelength of the input oscillation light, and a photodetector 53 that receives the transmitted light of the broadband wavelength filter 52 are provided.
[0079]
That is, wavelength information is converted into intensity information by the wavelength filters 50 and 52, and optical signals from the wavelength filters 50 and 52 are converted into electric signals by the photodetectors 51 and 53 according to the wavelength. A strong electrical signal can be obtained
FIG. 12 shows the wavelength characteristics of the narrow-band wavelength filter 50 and the wavelength characteristics of the broadband wavelength filter 52. Δλld is a wavelength variable region of the semiconductor laser, Δλf1 is a wavelength discrimination region of the broadband wavelength filter 52, and Δλf2 is a wavelength discrimination region of the narrowband wavelength filter 50. Further, the wavelength (channel) interval of the ITU grid is set to be equal to twice the wavelength discrimination region of the narrowband wavelength filter 50 (2 × λf2). The ITU grid is a set of closely spaced wavelengths in a specific wavelength region specified by the International Telecommunication Union, for example, a 1550 nm window, for example, a wavelength of about 0.8 nm for a 100 GHZ interval. Corresponds to the interval.
[0080]
In this type of SSG DBR-LD, the wavelength tunable region (Δλld) is as wide as 30 nm to 40 nm as compared with a normal semiconductor laser. The narrowband wavelength filter 50 has the advantage that the signal intensity change with respect to the wavelength change is large and the wavelength can be detected with high accuracy. However, since the wavelength discrimination region (Δλf2) of the narrowband wavelength filter 50 is narrow, only the narrowband wavelength filter 50 is available. In this case, it is impossible to cover the entire wavelength variable region (Δλld) of the semiconductor laser, and it is impossible to detect the absolute wavelength. Therefore, the entire wavelength variable region (Δλld) of the semiconductor laser is covered by the broadband wavelength filter 52 having a wavelength discrimination region (Δλf1) wider than Δλld, so that the grid is located, that is, the absolute wavelength of the oscillation wavelength. To detect.
[0081]
When using only the characteristic of one slope of the narrowband wavelength filter,
ITU grid wavelength interval = 2 × Δλf2
2 × Δλf2 <Δλld <Δλf1
The wavelength characteristics of the wavelength filters 50 and 52 are set so that the above relationship is established.
[0082]
In addition, when using the characteristics of both slopes of the narrowband wavelength filter,
ITU grid wavelength interval = Δλf2
2 × Δλf2 <Δλld <Δλf1
The wavelength characteristics of the wavelength filters 50 and 52 are set so that the above relationship is established.
[0083]
The wavelength control operation using the wavelength monitor 7 of FIG. 11 will be described with reference to FIG. First, the wavelength control circuit 21 in FIG. 1 acquires the output of the photodetector 53 on the broadband wavelength filter 52 side (step 300), and determines whether or not the target wavelength λc has been reached based on this output (step 300). Step 310). If the target wavelength λc has not been reached, the Peltier element 3 is driven via the power supply controller 26 to adjust the element temperature Tld to stabilize the target wavelength λc (step 320).
[0084]
When it is detected in step 310 that the target wavelength λc has been reached, the wavelength control circuit 21 acquires the output of the photodetector 51 on the narrowband wavelength filter 50 side (step 330). It is determined whether or not the target wavelength λc is stable based on the output (step 340). The wavelength control circuit 21 stabilizes the target wavelength λc by driving the Peltier element 3 via the power supply controller 26 and adjusting the element temperature Tld when the amount of wavelength deviation is larger than the predetermined oscillation wavelength accuracy. (Step 350).
[0085]
In this way, in the wavelength monitor 7 shown in FIG. 11, the oscillation wavelength is roughly adjusted based on the output on the wideband wavelength filter 52 side, and then the oscillation wavelength is finely adjusted on the basis of the output on the narrowband wavelength filter 50 side. By performing the above, even when the wavelength variable region of the semiconductor laser is large, highly accurate wavelength control can be performed.
[0086]
In the above embodiment, the SSG DBR-LD is used for the semiconductor laser 1. However, the present invention is also applied to an LD having another SG-DBR LD (Sampled Grating DBR) structure. be able to. In this case, the same effect as described above can be obtained. This SG-DBR LD is obtained by arranging a region having a diffraction grating having the same period and a region having no diffraction grating as one unit, and arranging them as an integer number of times. The lines are arranged at the same interval. Also in the case of SG-DBR LD, basically, the period of the reflection peak between the front SSG DBR area and the rear SSG DBR area is made different by changing the arrangement period between the front SSG DBR area and the rear SSG DBR area. ing. In the case of this SG-DBR LD, unlike the SSG-DBR, the difference in the period of the reflection peak can be achieved by changing the distance of the region without the grating, so that it is not necessary to change the period of the single grating. Note that the difference in the period of the reflection peak may be realized by changing the period of the single grating.
[0087]
In each of the above embodiments, a part of the front surface output light from the semiconductor laser 1 is input to the wavelength monitor 7, the oscillation mode monitor 8 and the light intensity monitor 9 as monitor light, but output from the back surface of the semiconductor laser 1. A small amount of oscillation light may be used as monitor light. In that case, it can be used as an external output signal without reducing the front light output. Further, the beam splitter 4 is not necessary, and the configuration is simplified.
[0088]
【The invention's effect】
As described above, according to the wavelength control device for a tunable semiconductor laser according to the present invention, a predetermined function capable of extracting only the conditions for stable oscillation is set in the database, and the resonator is configured using this function. The injection currents to the first and second light reflectors are controlled, so that highly accurate and stable wavelength control can be realized using a simple database with little stored data.
[0089]
According to the wavelength control method for a wavelength tunable semiconductor laser according to the next invention, the first and second light reflections constituting the resonator using a database storing a predetermined function capable of extracting only the conditions for stable oscillation are stored. In this way, it is possible to control the injection current to the device, and to achieve highly accurate and stable wavelength control using a simple database with little stored data.
[0090]
According to the wavelength tunable semiconductor laser device of the next invention, the injection current to the first and second optical reflectors constituting the resonator is controlled using the data stored in the simplified database, and the oscillation mode Since the injection current to the phase injection region is controlled based on the output of the monitor, more accurate and stable wavelength control can be realized using a simple database with little stored data.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a wavelength tunable semiconductor laser device to which an embodiment of the present invention is applied.
FIG. 2 is a cross-sectional view showing a configuration of an SSG DBR LD.
FIG. 3 is a graph showing wavelength characteristics of gain or loss in each region of SSG DBR LD.
FIG. 4 is a diagram showing If and Ir characteristics of oscillation wavelength in SSG DBR LD.
5 is a diagram clarifying the wavelength boundary region of the If and Ir characteristics of the oscillation wavelength of FIG. 4;
FIG. 6 is a diagram showing wavelength spectra of single longitudinal mode and unstable mode oscillation in SSG DBR LD.
FIG. 7 is a flowchart showing a wavelength tuning procedure of the wavelength tunable semiconductor laser according to the embodiment of the present invention.
FIG. 8 is a diagram for explaining functions used in a database.
FIG. 9 is a diagram illustrating an If characteristic of an oscillation wavelength in an SSG DBR LD.
FIG. 10 is a flowchart showing a procedure for determining If and Ir in a wavelength tuning procedure.
FIG. 11 is a block diagram showing another form of the wavelength monitor.
12 is a diagram illustrating wavelength characteristics of a wideband wavelength filter and a narrowband wavelength filter used in the wavelength monitor of FIG.
13 is a flowchart showing a procedure of wavelength control using the wavelength monitor of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Semiconductor laser, 2 Heat sink, 3 Peltier element, 4, 5, 6 Beam splitter, 7 Wavelength monitor, 8 Oscillation mode monitor, 9 Light intensity monitor, 10 Temperature monitor, 20 Semiconductor laser drive device, 21 Wavelength control circuit, 22 Oscillation Mode control circuit, 23 Light intensity control circuit, 24 Temperature control circuit, 25 Database, 26 Power supply controller, 31 Front SSG DBR region, 32 Active layer region, 33 Phase control region, 34 Rear SSG DBR region, 50 Narrow band wavelength filter, 51,53 Photodetector, 52 Broadband wavelength filter.

Claims (17)

Variable wavelength for controlling the oscillation wavelength of a tunable semiconductor laser having first and second optical reflectors having a plurality of reflection peaks and an active layer region disposed between the first and second optical reflectors. In a semiconductor laser wavelength control device,
A function that passes through one or more regions that stably oscillate from the relationship between the injection current to the first light reflector and the injection current to the second light reflector and the oscillation wavelength, or information for defining the function. Remembered database,
An injection current value to the first and second optical reflectors corresponding to a target wavelength is acquired based on the data stored in the database, and the first and second light reflections are acquired using the acquired injection current value. A power supply controller that controls the injection current
With
The above database has the following functions or constants a, b, and a constant for defining the following functions when the injection current to the first light reflector is If and the injection current to the second light reflector is Ir. A wavelength control device for a wavelength tunable semiconductor laser, comprising one set to a plurality of sets of data for c and d.
Ir = a × If + b
If = (c × λ + d) ² (λ: wavelength, a, b, c, d: constant) The wavelength tunable semiconductor laser, the wavelength tunable semiconductor laser according to claim 1, characterized in that the first and second optical reflector having a DBR structure using heterogeneous gratings having a period different lattice spacing Wavelength control device. The wavelength tunable semiconductor laser has a phase control region between the active layer region and the first or second optical reflector,
An oscillation mode monitor for monitoring the oscillation mode is further provided.
3. The wavelength control device for a wavelength tunable semiconductor laser according to claim 1, further comprising an oscillation mode control circuit that controls an injection current to the phase control region based on an output of the oscillation mode monitor. 4. The wavelength of the wavelength tunable semiconductor laser according to claim 3 , wherein the oscillation mode monitor determines whether the oscillation mode is single longitudinal mode oscillation or multimode oscillation based on an alternating current component of oscillation light. Control device. Further equipped with a wavelength monitor for monitoring the oscillation wavelength,
The wavelength tunable semiconductor according to any one of claims 1 to 4 , further comprising a wavelength control circuit that controls an element temperature or an injection current of the wavelength tunable semiconductor laser based on an output of the wavelength monitor. Laser wavelength control device. The wavelength monitor
A wavelength filter whose transmittance changes according to the wavelength of the input oscillation light;
A photodetector for receiving light transmitted through the wavelength filter;
The wavelength control device for a wavelength tunable semiconductor laser according to claim 5 , comprising: The wavelength monitor
A narrow-band wavelength filter whose transmittance changes according to the wavelength of the input oscillation light;
A first photodetector for receiving light transmitted through the narrow-band wavelength filter;
A broadband wavelength filter whose transmittance changes according to the wavelength of the input oscillation light,
A second photodetector for receiving light transmitted through the broadband wavelength filter;
The wavelength control device for a wavelength tunable semiconductor laser according to claim 5 , comprising: The wavelength discrimination region of the narrowband wavelength filter corresponds to the ITU grid,
8. The wavelength control device for a wavelength tunable semiconductor laser according to claim 7 , wherein a wavelength discrimination region of the broadband wavelength filter is larger than a wavelength tunable range of the semiconductor laser. 9. The wavelength control device for a wavelength tunable semiconductor laser according to claim 6 , wherein the wavelength filter is any one of a Fabry-Perot etalon, a birefringence filter, a multilayer filter, and a fiber grating. . It further includes a light intensity monitor that detects the light intensity of the oscillation light,
The wavelength according to any one of claims 1 to 9 , further comprising a light intensity control circuit that controls an injection current to the active layer region so that a detection output of the light intensity monitor is constant. Wavelength control device for variable semiconductor laser. An oscillation wavelength of a wavelength tunable semiconductor laser having first and second optical reflectors having a plurality of reflection peaks and an active layer region and a phase control region disposed between the first and second optical reflectors. In the wavelength control method of the tunable semiconductor laser to be controlled,
A function that passes through one or more regions that stably oscillate from the relationship between the injection current to the first light reflector and the injection current to the second light reflector and the oscillation wavelength, or information for defining the function. An injection current value to the first and second optical reflectors corresponding to a target wavelength is acquired based on the stored data of the stored database, and the first and second lights are acquired using the acquired injection current value. A light reflector control step for controlling the injection current to the reflector,
The above database has the following functions or constants a, b, and a constant for defining the following functions when the injection current to the first light reflector is If and the injection current to the second light reflector is Ir. A wavelength control method for a wavelength tunable semiconductor laser, comprising one to a plurality of sets of data for c and d.
Ir = a × If + b
If = (c × λ + d) ² (λ: wavelength, a, b, c, d: constant) The wavelength tunable semiconductor according to claim 11 , further comprising an oscillation mode control step that is executed after the optical reflector control step and controls an injection current to the phase control region based on the detected oscillation mode. Laser wavelength control method. The wavelength according to claim 12 , further comprising a wavelength control step that is executed after the optical reflector control step and controls an element temperature or an injection current of the wavelength tunable semiconductor laser based on a detected oscillation wavelength. A wavelength control method for a tunable semiconductor laser. 14. The wavelength control method for a tunable semiconductor laser according to claim 13 , further comprising a light intensity control step for controlling an injection current into the active layer region so that the detected light intensity is constant. A tunable semiconductor laser having first and second optical reflectors having a plurality of reflection peaks and an active layer region and a phase control region disposed between the first and second optical reflectors;
A function that passes through one or more regions that stably oscillate from the relationship between the injection current to the first light reflector and the injection current to the second light reflector and the oscillation wavelength, or information for defining the function. Remembered database,
An oscillation mode monitor for monitoring the oscillation mode;
An injection current value to the first and second optical reflectors corresponding to a target wavelength is acquired based on the data stored in the database, and the first and second light reflections are acquired using the acquired injection current value. A power supply controller that controls the injection current
An oscillation mode control circuit for controlling an injection current to the phase control region based on an output of the oscillation mode monitor;
With
The above database has the following functions or constants a, b, and a constant for defining the following functions when the injection current to the first light reflector is If and the injection current to the second light reflector is Ir. A wavelength tunable semiconductor laser device having one set to a plurality of sets of data for c and d.
Ir = a × If + b
If = (c × λ + d) ² (λ: wavelength, a, b, c, d: constant) 16. The optical branching device according to claim 15 , further comprising an optical branching device for branching a laser beam on an optical axis of a front light output of the wavelength tunable semiconductor laser and inputting one of the branched outputs to the oscillation mode monitor. Tunable semiconductor laser device. 16. The wavelength tunable semiconductor laser device according to claim 15 , wherein the oscillation mode monitor is arranged to receive part or all of the back light of the wavelength tunable semiconductor laser.

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