JPS6345877A - Method for stabilizing multiplicity of light source frequencies - Google Patents

Abstract

PURPOSE:To simultaneously stabilize a multiplicity of light source frequencies by a method wherein optical signals of various frequencies are directly and simultaneously put into a sweep-type interferometer, the output of the interferometer is detected by a photodetector, and differences between the optical frequencies are determined from a stabilized optical frequency. CONSTITUTION:The output of an optical fiber F16 travels through a lens L11 and then a Fabry-Perot etalon 30, and is coupled to a photodiode D11 by a lens L12. The fluctuation in optical frequency in a semiconductor laser 11 is converted into an optical intensity fluctuation by the etalon 30 and then into a signal intensity fluctuation by the photodiode D11. The signal intensity fluctuation is negatively fed back to the semiconductor laser 11 through a current control circuit 31, for the optical frequency to be absolutely stabilized. The output of a wave combining unit 29 travels through an optical directional coupler C12, optical fiber F19, lens L13, and then a sweep-type Fabry-Perot interferometer 32, to be coupled to a photodiode D12 by a lens L14 for direct detection. The output then travels through an A.D converter 33, and a processor 34 determines the relative differences between optical frequencies f12, f13, f14 and optical frequency f11, respectively. The differences are negatively fed back, respectively, to semiconductor lasers 22, 23, and 24 through a scanner 45 and D.A converter 36.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for stabilizing multiple light source frequencies for collectively stabilizing the oscillation frequencies of multiple lasers. The present invention is used as a light source for optical frequency division multiplexing transmission.

[Conventional technology]

FIG. 5 is a block diagram showing an example of a conventional light source using a plurality of light source frequency stabilization methods (Yi, Jie, Bachus et al. "Coherent Optical Fiber Subscriber Line" Electronics Letter; E. J. Bachus et al.
rCoherent optical-fiber 5u
bscriber1ineJ Electronics
(See Letters Vol. 21, p. 1203, 1985). This conventional example uses three semiconductor lasers 1, 2 and 3.
This shows a method for stabilizing the optical frequencies f+, rz, r-r of the optical signals. The stabilization method will be described next. First, the output of the control semiconductor laser 3 is coupled to the output optical fiber F3, which is branched into optical fibers F4 and F5, and the output of the optical fiber F5 is input to the fV converter 5. The r-v converter 5 has an optical output (V) characteristic as shown in FIG. 6 with respect to the input laser frequency (f). Such characteristics can be obtained, for example, by using a Fabry-Perot etalon or a ring resonator. At point A in FIG. 6, fluctuations in optical frequency are converted into fluctuations in output intensity. The optical frequency f of the semiconductor laser 3 is stabilized as an absolute frequency by detecting this with a photodiode 6 and feeding it negative feedback to a bias current or temperature through a control circuit 7.

The outputs of the semiconductor lasers 1.2 and 3 are coupled to a multiplexer 4 through output optical fibers F1, F2 and F4, and after multiplexing, are coupled to an optical fiber F6.

The output of optical fiber F6 is further transmitted to optical fiber F7 for transmission.
and is branched into a monitoring optical fiber F8. The output of the optical fiber F8 is mixed with the output of the control semiconductor laser 10 by the optical directional coupler C1, and is subjected to hetero gain detection by the photodiode 11. Now, when the optical frequency fc of the semiconductor laser 10 is swept over the optical frequencies f, , ft, and F3 by the sawtooth wave generation circuit 9, fc = f
When n (n=1.2.3), the semiconductor laser 10 generates a hetero gain detection output pulse.

The channel frequency control circuit 8 stores the generation time of each output pulse, compares it with the set value of the relative frequency position between each laser, and sets a temperature calibration value T+ corresponding to the difference.
Tz and Tc are semiconductor lasers 1.2 and 1, respectively.
By feeding back to the zero temperature, each oscillation frequency is stabilized.

[Problem that the invention seeks to solve]

In the conventional stabilization method described above, heterodyne detection is performed in the semiconductor laser 10, so the detection circuit becomes complicated and it is necessary to align the polarizations between the lasers. When a conventional single mode fiber is used, disturbances such as temperature cause polarization fluctuations, which may lead to deterioration in frequency measurement accuracy. Therefore, it becomes necessary to introduce an expensive polarization-maintaining fiber in order to suppress polarization fluctuations. Also,
When the number of multiplexed signals increases, it becomes necessary to sweep the optical frequency fc over a wide range. For example, when multiplexing 10 waves at a frequency interval of 5 GHz, the minimum required sweep frequency is 45 GHz.
Hz. Typical wavelength 1.55. In order to change Iz to 45G) in an InP-based DFB laser from n-band InGa8sP, it is necessary to change the temperature by 4°C, and it takes more than 1 minute to sweep once, causing frequency fluctuations. It has the disadvantage that it cannot be suppressed fast enough.

Moreover, the oscillation frequency can also be changed by the bias current of the semiconductor laser. In this case, the oscillation frequency is 45
A change in GHz corresponds to a change in current of approximately 45 mA, which causes the oscillation output of the semiconductor laser 10 to fluctuate greatly, degrading frequency measurement accuracy. As described above, the conventional stabilization method has the disadvantage that it does not function satisfactorily when the number of multiplexes is increased.

An object of the present invention is to provide a method for stabilizing multiple light source frequencies that can stabilize multiple light source frequencies at once with a simple and stable configuration by eliminating the above-mentioned drawbacks. be.

[Means for solving problems]

The present invention stabilizes the optical frequency of any one of the plurality of lasers that generate optical signals of different optical frequencies using a stabilizing circuit, and combines the optical signal of this stabilized optical frequency with the optical signals of each of the other lasers. The optical signals are combined, the optical frequencies of the other lasers are compared using the optical frequency of the stabilized laser by a predetermined means, the amount of deviation from the set value is detected, and the amount of deviation from the set value is detected. In a method for stabilizing multiple light source frequencies in which the optical signal generation conditions of each laser are controlled and the optical frequency is stabilized, a sweep interferometer is used as the predetermined means, and the optical frequency of the stabilized laser and the It is characterized by measuring the distance from the optical frequency of each other laser and comparing that value with each set value to determine the amount of deviation.

[For production]

Swept type interferometer For example, swept type Fabry-Perot interferometer,
Multi-wave optical signals are directly input simultaneously and interfered optical signals corresponding to each optical frequency are output. Therefore, by detecting this output with a light receiving element, determining the frequency interval of each optical frequency using the stabilized optical frequency as a reference, and comparing this value with a preset value, the deviation amount of each optical frequency can be obtained. .

That is, unlike the conventional example, a special control laser (fifth
The semiconductor laser 10) shown in the figure is not required, and the amount of deviation of each optical frequency can be obtained directly rather than indirectly by hetero gain detection or the like.

Therefore, it is possible to stabilize a plurality of light source frequencies at once with a simple and stable configuration.

〔Example〕

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

FIG. 1 is a block diagram showing a light source according to an embodiment of the present invention, and shows the case of four semiconductor lasers.

The present embodiment includes four semiconductor lasers 21, 22, 23 and 24 having single-longitudinal mode spectra, respective temperature stabilization circuits 25, 26, 27 and 28, and respective output optical fibers Fil, F12, F1a and F14 and the optical fiber Fil are connected to the optical fiber 15 and F14.
Optical directional coupling RSCII which branches into 16 and optical fibers F15, F12, F1a and F14 are connected to the input, and optical frequencies f0, f12, f12,
a multiplexer 29 that multiplexes the optical signals of f13 and F1a;
Lenses Lll, L12, Fabry-Perot etalon 30, and photodiode Dll for discriminating the optical frequency of the semiconductor laser 11 branched to the optical fiber F16
, a current control circuit 31 connected to the output of the photodiode Dll and controlling the bias current of the semiconductor laser 11, an optical fiber 17 for outputting the multiplexer 29, and an optical fiber F18 for transmitting the optical fiber 17. An optical directional coupler C12 branches into an optical fiber F19 for monitoring, and a sweep-type Fabric coupler C12 that inputs the optical signal from the optical fiber F19 through a lens L13 and measures the frequency interval of each optical frequency.
Perot interferometer 32 and swept Fabry-Perot interferometer 3
A lens L14, a photodiode D12, and a photodiode D1 take out the output of 2 and convert it into an electrical signal.
An A-D converter 33 inputs the output of 2 and converts it into a digital signal, and an A-D converter 33 inputs the output of the A-D converter 33 and calculates the amount of deviation of each optical frequency f18, [13 and F1a from a predetermined setting value. a scanner 35 that distributes the output of the processor 34 to each of the semiconductor lasers 12, 13 and 14; and a scanner 35 that converts the output of the scanner 35 into an analog signal and uses it as a bias current for each of the semiconductor lasers 12, 13 and 14. It includes a DA converter 36 that provides negative feedback. Note that all optical fibers here are of single mode.

Next, the operation of this embodiment will be explained. First, temperature stabilization of the semiconductor lasers 21-24 is performed by respective temperature stabilization circuits 25-28. Each optical signal is coupled to the optical fibers Fil to F14, and is optically multiplexed and coupled to the optical fiber F17 by a multiplexer 29. At this time, the output of the optical fiber Fil is branched by an optical directional coupler C1l into an optical fiber F15 and an optical fiber F16, which are coupled to a multiplexer 29. The output of optical fiber F16 is collimated by lens Lll, transmitted through Fabry-Perot etalon 30, and coupled to photodiode Dll by lens L12. Fabbri Perrault etalon 30
Accordingly, the optical frequency fluctuation of the semiconductor laser 11 is converted into a light intensity fluctuation, which is further converted into a signal intensity fluctuation by the photodiode DLL. By feeding this back negatively to the bias current of the semiconductor laser 11 through the current control circuit 31, the optical frequency of the semiconductor laser 11 is absolutely stabilized. When using an etalon with a reflectance of 90% at both ends and a thickness of 51 mm, a frequency stability of about I MHz was experimentally obtained (Satoshi Toba, "Study of a light wave transmission LD light source frequency stabilization circuit", 1985 IEICE General Conference Proceedings 264
(see 7). The semiconductor laser used in the experiment has a wavelength of 1.5
This is an InGaAs1'/InP distributed feedback semiconductor laser that oscillates in the − band. In this case, the amount of variation in the oscillation frequency with respect to the current is typically IGHz/edge.

On the other hand, the optical frequencies of the semiconductor lasers 22 to 24 are stabilized relative to the optical frequency of the semiconductor laser 11 using the swept Fabry-Perot interferometer 32 as a frequency monitor.

The procedure will be described below. Optical fiber F1 for multiplexer output
7 is an optical fiber F for transmission by an optical directional coupler C12.
18 and a monitoring optical fiber F19, and the output of the optical fiber F19 is collimated by a lens L13, transmitted through a swept Fabry-Perot interferometer 32, and coupled to a photodiode D12 by a lens L14.
It is directly detected by the photodiode D12.

FIG. 2 is a characteristic diagram showing the output vector of the swept Fabry-Perot interferometer 32. In FIG. 2, fll to f14 are the optical frequencies of the semiconductor lasers 11 to 14, respectively, and the frequency interval of each semiconductor laser is 5 GHz.

The free space length of the Fabry-Perot interferometer 32 is 30 GHz
It is set to . The output of photodiode D12 is A-
The optical frequency f is converted by the processor 34 through the D converter 33.
The relative frequency differences between 1□, f13, and f14 and the optical frequency r are each measured, and the error from the predetermined frequency difference is calculated. Thereafter, the amount of current corresponding to the frequency error is transmitted to each semiconductor laser 22 through the scanner 35 and the D-A converter 36.
By providing negative feedback to the bias current of the semiconductor laser 24, the optical frequencies r+z~f14 of the semiconductor lasers 22~24 can be stabilized relative to the optical frequency f of the semiconductor laser 21, and as a result, the optical frequencies r+z~f14 of the semiconductor lasers 22~24 can be stabilized relative to the optical frequency f of the semiconductor laser 21. 24 optical frequencies fll to f14 are absolutely stabilized.

FIG. 3 is a characteristic diagram showing temporal changes in relative frequency when stabilization is not performed. A drift of about 600 MHz occurs in 15 minutes.

FIG. 4 is a characteristic diagram showing temporal changes in relative frequency during frequency stabilization. The current feedback interval is 2 seconds, and the short-term fluctuation (52 seconds) is 12 (1 MHz) when stabilized compared to 140 MHz (52 seconds) before stabilization, although no significant improvement is seen.
Long-term drift is almost suppressed.

Short-term stability is also improved by increasing the current feedback rate.

Further, in this embodiment, feedback is made to the bias current of the semiconductor laser, but it is also possible to feed back to the lens temperature to stabilize it. InGaAsP/InP oscillating in the 1.5-band
The amount of variation in oscillation frequency with respect to temperature of a distributed feedback semiconductor laser is about 10 GH2/'C.

Furthermore, although the four optical frequencies were stabilized in this example,
Frequencies above this can be similarly stabilized simply by setting the free space length of the swept Fabry-Perot interferometer to be larger than the entire frequency range.

The feature of the present invention, as shown in FIG. 1, is that the optical frequencies of a plurality of light sources are measured at once using a sweeping interferometer and are stabilized at the same time. In the conventional technology, the frequency difference between each light source is measured by hetero gain detection, whereas the present invention is different in that direct detection is performed and a control laser (semiconductor laser 10 in FIG. 5) is not used.

〔Effect of the invention〕

As explained above, the present invention is capable of stabilizing multiple light source frequencies at once, and also directly detects each light source frequency. Moreover, the effect can be achieved with a stable configuration.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a light source according to an embodiment of the present invention. FIG. 2 is a characteristic diagram showing the output vector of the swept Fabry-Perot interferometer during stabilization as shown in FIG. FIG. 3 is a characteristic diagram showing the relative frequency change over time when stabilization is not performed in FIG. 1. FIG. 4 is a characteristic diagram showing temporal changes in relative frequency when stabilization is performed in FIG. 1. FIG. 5 is a block diagram showing a conventional light source. FIG. 6 is a characteristic diagram showing the relationship between optical frequency and optical output of the fV converter of FIG. 5. 1-3.10.21-24... semiconductor laser, 4.2
9... Multiplexer, 5... fV converter, 6.11...
- Photodiode, 7... Control circuit, 8... Channel frequency control circuit, 9... Sawtooth wave generator, 2
5-28... Temperature stabilization circuit, 30... Fabry
Perot etalon, 31...Current control circuit, 32...
Sweeping Fabry-Perot interferometer, 33...A-D converter, 34...Processor, 35...Scanner, 36
... D-A converter, C1, C1l, C12... Optical directional coupler, Dll, D12... Photodiode, F1 to F8, Fil to F19... Optical fiber, f,
~f1, f11~f14... optical frequency, Lll~L1
4...Lens, 71%T2, Tc...Temperature calibration value.

Claims (1)

[Claims] (1) For multiple lasers that generate optical signals with different optical frequencies, the optical frequency of any one laser is stabilized by a stabilizing circuit, and the optical signal of this stabilized optical frequency is combined with the optical signal of each other laser. The signals are combined, and the optical frequencies of the other lasers are compared using the optical frequency of the stabilized laser by a predetermined means to detect the amount of deviation from the set value, and the amount of deviation from the set value is detected. In a plurality of light source frequency stabilization methods that control the optical signal generation conditions of a laser and stabilize the optical frequency, a sweeping interferometer is used as the above-mentioned prescribed means, and the optical frequency of the stabilized laser and the above-mentioned others are A method for stabilizing a plurality of light source frequencies, the method comprising: measuring the interval between the optical frequencies of each laser, and comparing that value with each set value to determine the amount of deviation.