JPH10321938A - Frequency stabilized light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize an oscillation frequency, by fixing equivalent reflactivities of a front and a rear distribution reflector region to the same value and laser oscillation wavelengths to one wavelength, so that Bragg wavelengths of the front and the rear distribution reflectors may coincide. SOLUTION: Bias current sources 18-21 are connected to a front side distribution reflector region(DBR) 2, a rear side DBR 3, a phase adjusting region 4, and a gain region of an SSG-DBR laser (laser) 1, respectively. The DBRs 2 and 3 are added with sine wave signals from signal sources 16, 17. The output light from the laser 1 is branched by means of fiber couplers 7, 8 and part of the light is guided to a synchronous detector 23 through an optical detector 9. The synchronous detector 23 makes a phase synchronous detection using the signal sources 16, 17, and detects an error A between a reflection peak wavelength and a longitudinal mode wavelength of the front side DBR and an error B between a reflection peak wavelength and a longitudinal mode wavelength of the rear side DBR. The detected errors A and B are fed back to the DBRs 2 and 3 through an amplifier 14 and a low pass filter 15 to stabilize the laser oscillation frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention fixes the wavelength (optical frequency) of output light of a semiconductor laser to a constant value,
The present invention relates to a frequency-stabilized light source for obtaining laser light having a stable wavelength of the semiconductor laser.

[0002]

2. Description of the Related Art At present, research on an optical frequency (wavelength) multiplex communication system has been actively conducted in order to increase the amount of communication information. In this case, a semiconductor laser is mainly used as a light source for transmission, and a semiconductor laser having a single mode oscillation characteristic is required for frequency multiplexing. Moreover, in the case of using a light source capable of greatly changing the oscillation frequency, it is possible to access an arbitrary wavelength channel of the wavelength division multiplex communication network, so that the wavelength division multiplex network can be rearranged.
It is possible to construct a sophisticated wavelength multiplex communication network.
When such a semiconductor laser is used, as a light source used for wavelength division multiplexing communication, it is important that the optical frequency is fixed at a constant value and stable for a long time.

[0003] Distributed feedback (DFB) semiconductor lasers are currently the mainstream of light sources for long-distance optical communication systems because of their excellent single-mode oscillation characteristics. ) Although it is expected as a light source for a multiplex communication system, the oscillation frequency cannot be largely changed. On the other hand, a distributed reflection (DBR) semiconductor laser is
By injecting a current into the distributed reflector region, it is possible to greatly change the wavelength as compared with a DFB laser.
In particular, a four-electrode SSG-DBR laser in which a super-periodic structure diffraction grating is formed on the front and rear distributed reflectors is 30-6
Since it is a single mode semiconductor laser having a wavelength tunable range of 0 nm, it is very promising as a light source for wavelength multiplexing.

The oscillation frequency of the semiconductor laser is not limited to the DFB type and the DBR type, and the oscillation frequency of the semiconductor laser greatly changes depending on the temperature. Therefore, temperature control for the oscillation frequency is necessary. Under typical use, the oscillation frequency also fluctuates due to fluctuations in element characteristics such as deterioration of the active region, so that a control circuit for further stabilizing some frequency (wavelength) is required.

FIG. 4 is an example of a configuration diagram of a frequency stabilized light source according to a conventional method using a four-electrode SSG-DBR laser. The SSG-DBR laser 1 has a front SSG-DB
It has an R region 2, a rear SSG-DBR region 3, a phase adjustment region 4, and a gain region 5, and each region is provided with an electrode. These electrodes are connected to different bias current sources 18, 19, 20, and 21, respectively, and the approximate oscillation wavelength is determined by the bias current value. Part of the output light is incident on a cross-discrimination type frequency reference filter 11, and a differential amplifier 12 detects the amount of deviation between the reference frequency and the frequency of the laser light. The amount of this shift is used as the phase adjustment area 4
The oscillation frequency is stabilized at the reference frequency of the filter by feeding back the current flowing through the filter.

[0006]

However, in the above prior art, although a certain degree of stability is obtained,
There was a problem of lack of long-term stability. SSG-
For lasers with distributed reflectors before and after such as DBR lasers,
From a number of resonance longitudinal modes determined by the laser resonator, only one longitudinal mode is selected by a sharp reflection peak of distributed reflection before and after, and oscillation is performed in a single mode. And
By changing the wavelength of the reflection peak by current injection and selecting an arbitrary longitudinal mode, the oscillation wavelength can be largely changed. On the other hand, a stable single mode operation cannot be obtained unless the reflection peaks of the front and rear distributed reflectors always match the longitudinal mode. In the above-mentioned conventional example, although the longitudinal mode wavelength is controlled to match the reference wavelength of the filter by adjusting the current flowing through the phase adjustment region 4, the front and rear reflection peaks and the longitudinal mode are different for a long time. There was a problem that there was no guarantee that they did. For example, if the laser is used for a long time, the portion of the distributed active region is degraded, and the optical refractive index of the portion is changed,
There is a risk that the oscillation longitudinal mode jumps to another mode. In such a situation, it becomes impossible to tune the frequency to the reference filter with the conventional method.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems in the prior art, and to obtain a light source using a four-electrode distributed reflection (DBR) laser whose oscillation frequency is always stable against long-term fluctuations in device characteristics. That is.

[0008]

The present invention for achieving the above object has the following specific features. (1) A four-region distributed reflection type semiconductor laser including a gain region, a phase adjustment region, and two distributed reflector regions before and after, and the Bragg wavelengths of the two distributed reflectors match the oscillation wavelength of this laser. Thus, it has a function of adjusting the equivalent refractive index of the two distributed reflector regions and a function of fixing the laser oscillation wavelength to one wavelength. (2) Front-side distributed reflector area with first electrode, second
A distributed reflector semiconductor laser having a rear-side distributed reflector region having a third electrode, and a phase adjustment region having a third electrode, and injecting a current intensity-modulated into the first electrode, A function of setting a current flowing to the first electrode so that a signal obtained by phase-locking detection of the output light of the laser is minimized, and injecting a current of minute intensity modulation into the second electrode, A function of setting a current flowing to the second electrode so that a signal obtained by phase-locking detection of the laser output light is minimized, and a wavelength of the laser output light matches a reference wavelength of the optical frequency reference device. As described above, a function of setting the current to be injected into the third electrode is provided. (3) In the above (2), a function of setting a current to be injected into the fourth electrode so as to have a fourth electrode in a gain region of the distributed-reflection semiconductor laser and to have a constant light output intensity of the laser. Having.

In the present invention, the amount of deviation between the front and rear reflection peak wavelengths and the longitudinal mode wavelength is detected by applying minute modulation signals having different frequencies to the front and rear DBR regions and synchronously detecting the optical output light. The control is performed such that the front and rear reflection peak wavelengths always coincide with the longitudinal mode. FIG.
A change in light intensity with respect to the front DBR current and a change in light intensity with respect to the rear DBR current are shown. FIG.
The curve in (a) is convex downward, and the curve in FIG. 2 (b) is convex upward. This indicates that when the reflectance of the front side distribution reflector becomes maximum, the light output becomes very small, and when the reflectance of the rear side distribution reflector becomes maximum, the light output becomes minimum. is there. Therefore, by detecting the optical output synchronously, the deviation amount from these extreme points is detected, this error signal is appropriately amplified, and fed back to the front and rear DBR regions, so that the longitudinal mode frequency and the Bragg frequency always match. Can be done.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Here, an embodiment of the present invention will be described with reference to FIGS. In FIG. 1, the same parts as those in FIG. 4 are denoted by the same reference numerals. SSG-DB consisting of 4 electrodes
Bias current sources 18, 19, 20, and 2 are provided in the front distributed reflector (DBR) region 2, the rear distributed reflector region 3, the phase adjustment (PC) region 4, and the gain region 5 of the R laser 1, respectively.
1 is connected and a current is injected. Signal sources 16 and 17 apply minute sine wave signals to the front and rear DBR regions. Here, the signal from the front DBR region 2 and the rear DBR
The frequency of the modulated signal is set to a different value in order to detect the signal from the region 3 separately. SSG-DBR
The optical output from the laser 1 is extracted by an optical fiber 6, and a part thereof is separated by fiber couplers 7 and 8 and guided to a photodetector 9. The signal converted into the electric signal by the photodetector 9 is divided into a front DBR region 2 and a rear DBR region 3.
Using the reference signals 16 and 17 added to the synchronous detector 2
3, and is detected as a shift amount A between the front DBR reflection peak wavelength and the longitudinal mode wavelength and a shift amount B between the rear DBR reflection peak wavelength and the longitudinal mode wavelength. The error signals A and B are appropriately amplified by the amplifier 14, respectively, passed through a low-pass filter 15 for ensuring negative feedback, and then fed back to the front and rear DBR regions 2 and 3, respectively. Is stabilized at the longitudinal mode wavelength.

A part of the light extracted by the fiber couplers 7 and 8 is input to a cross-discrimination type optical filter (for example, a volume holograph) 11 which is used as a reference for the optical frequency. In the next stage of the filter 11, two photodetectors 1 are provided.
0 is arranged and photoelectrically converted, and further becomes an input of the differential amplifier 12, and each input is obtained by taking a difference. This optical filter 11 has two output ports, the output characteristics of which show transmission characteristics having a peak at a slightly different optical frequency across the reference frequency, and the two outputs become equal at the reference frequency. Therefore, the output signal of the differential amplifier shows a value proportional to the amount of deviation from the reference frequency. By feeding this signal back to the phase adjustment current, the oscillation frequency (wavelength) is stabilized at the optical reference filter frequency (wavelength).

Further, in FIG. 1, in order to keep the intensity of the output light constant, a part of the output of
, Differentially amplifies the reference voltage, and feeds back this signal to the current flowing in the active region. Although not shown, the temperature of the laser is controlled to be constant using a Peltier element.

FIG. 3 is a graph showing the effect of the present invention.
It is a result of confirming by an experiment how an oscillation wavelength changes when a laser temperature is changed. In the figure, the symbol Δ indicates the case according to FIG. 4 in which the error signal between the reference optical filter wavelength and the reflection peak wavelength and the longitudinal mode were not stabilized, and the error signal from the reference optical filter wavelength was fed back to the phase adjustment region. Showing things. As is clear from FIG. 3, in the conventional method, when the temperature changes by about ± 1.5 ° C., the longitudinal mode wavelength jumps to the next mode, so that the oscillation wavelength immediately deviates from the stabilization range. On the other hand, according to the present embodiment, it is stabilized over a wide temperature range. This is because the control is performed so that the reflection peak wavelength always coincides with the longitudinal mode wavelength. This result indicates that the stabilization method according to the present embodiment is resistant to long-term changes in device characteristics.

Although a volume holograph is used as a reference optical frequency (wavelength) filter in FIG. 1, an interference type filter such as a Mach-Zehnder type filter or an array grating filter or a gas absorption cell is also used. Applicable. Further, in FIG. 1, a proportional control configuration is adopted in which the detected error signal is amplified, passed through a reduction pass filter, and then fed back. However, a so-called P.R. I. D. A control circuit may be inserted to increase the stability. Further, the present invention can be applied to a case where the detected error signal is converted into a digital signal by an A / D converter, and digital signals are processed using a computer to control each current value.

[0015]

As described above, according to the present invention, by stabilizing the front and rear reflection peak wavelengths and the longitudinal mode wavelength separately, it is possible to obtain output light having a stable frequency (wavelength) over a long period of time. it can.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating an embodiment of the present invention.

FIG. 2 shows light output (a) and rear D with respect to front DBR current.
The figure which shows the optical output (b) with respect to BR current.

FIG. 3 is a diagram comparing results between a conventional example and the present invention.

FIG. 4 is a configuration diagram of a conventional example.

[Explanation of symbols]

14 electrode super periodic structure diffraction grating distributed reflection type (SSG-D
BR) laser 2 front side distributed reflector (DBR) region 3 rear side distributed reflector (DBR) region 4 phase adjustment (PC) region 5 gain region 6 optical fiber 7,8 fiber coupler 9,10 photodetector (photodiode) Reference Signs List 11 optical frequency reference filter (volume holograph) 12, 13 differential amplifier 14 amplifier 15 low-pass filter 16, 17 reference signal source 18 bias current source to front distributed reflector region 19 bias current to rear distributed reflector region Source 20 Bias current source to phase adjustment region 21 Bias current source to gain region 22 Voltage source 23 Phase synchronous detection circuit

Claims (3)

[Claims] 1. A four-region distributed-reflection semiconductor laser comprising a gain region, a phase-adjusting region, and two distributed reflector regions before and after, and the Bragg wavelength of the two distributed reflectors is set to the oscillation wavelength of the laser. A frequency-stabilized light source having a function of adjusting the equivalent refractive index of the two distributed reflector regions so as to match, and a function of fixing the laser oscillation wavelength to one wavelength. 2. A front distributed reflector region with a first electrode, a rear distributed reflector region with a second electrode, and a third distributed reflector region.
A distributed-reflection semiconductor laser having a phase adjustment region with an electrode of the type described above, a current obtained by injecting a current whose intensity is minutely modulated into the first electrode, and a phase-locked detection of output light of the laser is obtained. A function of setting a current flowing to the first electrode so as to be a minimum, and a signal obtained by injecting a current whose intensity is minutely modulated into the second electrode and phase-locking detecting the output light of the laser to minimize the signal. And a function of setting a current to be injected into the third electrode so that the wavelength of the output light of the laser coincides with the reference wavelength of the optical frequency reference device. And a frequency stabilized light source. 3. A distributed reflection type semiconductor laser having a fourth electrode in a gain region and having a function of setting a current to be injected into the fourth electrode so that the light output intensity of the laser is constant. 2. The frequency stabilized light source according to 2.