JPH01259588A - Variable finesse type fabry-perot interferometer and oscillation frequency stabilization thereby - Google Patents

Abstract

PURPOSE:To improve frequency stability and broaden a linewidth on a frequency axis when the frequency is stabilized, by providing Q varying means interposed between a pair of reflecting mirrors with a polarizer, a Faraday rotator, and means for applying an arbitrarily determined magnetic field to the Faraday rotator. CONSTITUTION:An interferometer comprises a pair of reflecting mirrors 1, 2 and Q value varying means interposed therebetween. The Q varying means includes a polarizer 3, a Faraday rotator 4, and means 5 for applying an arbitrarily determined magnetic field to the Faraday rotator 4. When controlling the oscillation frequency of semiconductor laser 6 so that it will coincide with the frequency that provides a peak intensity 9 or a predetermined intensity 10 of the transmission spectrum of the finesse type Fabry-Perot interferometer, a relatively small finesse 11 is first used, and a relatively large finesse 12 is then used. Accordingly, a broadened linewidth can be achieved on a frequency axis, whereby a high frequency stability can be obtained.

Description

[Detailed description of the invention] Table of contents Summary ・ ・ ・ ・ ・ ・ ・ ・ ・
・ ・ ・ 2 pages Industrial application fields ・・・・・・・
・Page 3: Prior art ・・・Page 4: Problem to be solved by the invention ・Page 9: Means for solving the problem ・・Page 10: Effects ・・
・ ・ ・ ・ ・ ・ ・ ・ ・ ・ 12 pages actual
Example ・ ・ ・ ・ ・ ・ ・ ・ ・
・ ・ Page 14 Effects of the invention ・・・・・・・・・・・・2
2 page summary Concerning a finesse variable Fabry-Perot interferometer and a method for stabilizing the oscillation frequency of a semiconductor laser using the same, the purpose is to increase the frequency stability and expand the pull-in width on the frequency axis when performing frequency stabilization. It is composed of a pair of reflecting mirrors and a Q value variable means inserted between them, and the Q value variable means applies an arbitrary magnetic field to a polarizer, a Faraday rotator, and the Faraday rotator. It consists of means.

INDUSTRIAL APPLICATION FIELD The present invention relates to a variable finesse Fabry-Perot interferometer and a method for stabilizing the oscillation frequency of a semiconductor laser using the same.

In the field of optical communications or optical transmission using optical fibers as transmission paths, wavelength division multiplexing (WDM) transmission systems are effective in increasing the transmission capacity per single transmission path. In recent years, with the development of semiconductor lasers (LD) with narrow linewidth single longitudinal mode spectra such as DFB lasers,
High-density WDM transmission is now possible. As density increases and the wavelength spacing becomes less than IA, it is easier to understand the wavelength spacing as a frequency spacing, so in this specification, optical signals are multiplexed and transmitted at frequency spacings of several GHz to several tens of GHz. This method will be particularly referred to as an optical frequency division multiplexing (optical FDM) transmission method. When implementing this method, the transmitting light source is required to have an oscillation spectrum in a single longitudinal mode with a narrow linewidth and to have a stable center frequency over time, as described above. The present invention meets the latter of these requirements.

BACKGROUND ART FIG. 8 is an explanatory diagram of an optical FDM transmission system. Optical transmission 14
31-The output lights of frequencies f,, rb, f,,... are output from 31-a, b, c,..., respectively.
The signals are combined and sent to the optical transmission line 82. The optical FDM signal light transmitted through the optical transmission line 82 is transmitted through optical taps 83 and 84.
optical receivers 85, 86 .
... is received.

FIG. 9 is an explanatory diagram of the configuration side of a general optical receiver, (
A) shows a heterogain or homodyne detection method in a coherent optical communication system, and et al. shows a normal direct detection method. In (a), the optical FDM signal light transmitted through the optical transmission line 91 and the local oscillation light from the local oscillation light source 92 are mixed by a mixer 93 and input to a photodetector 94 such as a photodiode. At this time, the signal component of each optical FDM signal light has a frequency (
For example, since it is extracted as an intermediate frequency signal (in the case of veterodyne detection) of several GHz), an appropriate bandpass filter 9 is used.
5 can be separated into respective multiplex signal components. According to this method, it is expected that receiving sensitivity will improve, so it will not only be possible to increase the repeater interval, reduce the number of repeaters, or increase the number of branches in the optical transmission line, but also enable high-density frequency multiplexing. Therefore, it becomes possible to economically configure the optical transmission line. On the other hand, et al.
, the optical FDM signal light transmitted by the optical transmission line 101 is separated into respective signal lights at the optical stage by a high-precision optical demultiplexer 102, and the separated signal lights are each sent to a light receiving element 103-a. ,b.

C1 . . . and electric circuits IQ4-a, b, c, . . . convert it into each electrical signal component. This method can be applied as is to a normal intensity modulation method, making it possible to economically construct an optical transmission line.

When performing intensity modulation or other modulation on the transmitting side in response to these various reception methods, for example, in a mask station, it is required that the oscillation frequency of the light source is absolutely stabilized, and for example, in a slave station, It is required that the oscillation frequency of each light source be relatively stabilized. For absolute frequency stabilization, it is effective to use atomic or molecular absorption lines, etc. For relative frequency stabilization, it is effective to use periodic methods on the frequency axis such as Fabry-Perot interferometers and Matsuhatsu Enda interferometers. It is effective to use an interferometer that generates resonance. A frequency stabilization method using a Fabry-Perot interferometer will be described below.

FIG. 1O is a block diagram showing an example of a conventional LD oscillation frequency stabilizing device. The light emitted from the LDll driven by the bias circuit 120 is roughly divided into two parts by the optical coupler 112, one part is directly input to the first light receiver 113, and the other part is sent to the second light receiver via the Fabry-Perot interferometer 114. The light is input to the device 115. First and second light receivers 113.
The output of 115 is input to divider 116, and its output voltage Vo is compared with voltage output Vs from reference power supply 117 in comparator 118. Then, based on the output signal of the comparator 118, the control circuit 119 performs feedback control (for example, PID control) of the drive current of the LD 112.

FIG. 11 shows the output V of the divider. It is a graph which shows the relationship between and the oscillation frequency of LDIII. This resonance spectrum is
By keeping the temperature, etc. of the Fabry-Perot interferometer 114 constant, the oscillation frequency of LDIII can be obtained with extremely high reproducibility. can be stabilized to fs corresponding to Vs.

FIG. 12 is a block diagram of another conventional frequency stabilizing device, in which the oscillation frequency of the LD is stabilized at the peak of the transmission spectrum of the Fabry-Perot interferometer. The light emitted from the LD 131 passes through a Fabry-Perot interferometer 132 configured to mechanically vibrate the resonator length, and is received by a light receiver 133 , and the light receiving output is input to a lock-in amplifier 136 . 134 is a low frequency oscillator 13
This is a controller for mechanically vibrating the Fabry-Perot interferometer 132 based on the output of the Fabry-Perot interferometer 132. In the lock-in amplifier 136, the change in received light intensity is synchronously detected by the branched output of the low frequency oscillator 135, and the bias circuit 138 is controlled, for example, by PID, by the operation of the control circuit 137 based on the detected output.

In FIG. 13, 141 is the transmission spectrum of the Fabry-Perot interferometer 132, and 142 is its first-order differential curve, that is, the lock-in amplifier 13.
6 output characteristics. In this way, by vibrating the resonator length at a low frequency and performing synchronous detection, frequency discrimination can be performed using the linear part of the -th order differential curve 142, so the oscillation frequency of the LD 131 can be feedback-controlled. It is something that can be done.

Problems to be Solved by the Invention FIG. 14 shows the relationship between the transmitted intensity and frequency of a Fabry-Perot interferometer using the reflectance of the Fabry-Perot interferometer as a parameter. Transmission spectra 151a, b,
c is the value when the reflectance increases in this order, and it is clear that the spectrum peak becomes sharper as the reflectance increases. Here, the finesse F representing the sharpness of the spectral peak is expressed as F=(FSR)/W. FSR is the frequency interval between adjacent spectral peaks and corresponds to the free space length of this interferometer.

Further, W corresponds to the half width of the spectral peak.

By the way, in the frequency stabilization method (apparatus) shown in FIGS. 10 and 12, it is obvious from the control mode that the larger the finesse, the higher the frequency stability. However, if finesse is large,
When stabilizing the frequency of a free-running LD, the pull-in width on the frequency axis is narrow and adjustment work is difficult.
Further, there is a problem in that when frequency stabilization is performed, lock may be lost due to sudden frequency fluctuations. Therefore, in the past, it was necessary to select either one of allowing a low frequency stability to expand the pull-in width, or allowing a narrow pull-in width to improve the frequency stability.

The present invention was created in view of these circumstances, and aims to satisfy the requirements of increasing frequency stability and widening the pull-in width on the frequency axis when performing frequency stabilization.

Means to solve problems FIG. 1 is a diagram showing the principle of the present invention.

The finesse variable Fabry-Perot interferometer, as shown in FIG. Polarizer 3, Faraday rotator 4, and Faraday rotator 4
and means 5 for applying an arbitrary magnetic field to the magnetic field.

When using this variable finesse Fabry-Perot interferometer to stabilize the oscillation frequency of a semiconductor laser,
As shown in ), the light from the semiconductor laser 6 is transmitted through the variable finesse Fabry-Perot interferometer and received by the light receiver 7 ( ).

The oscillation frequency of the semiconductor laser 6 is shown in the same figure (C
), the peak intensity 9 or the predetermined intensity 10 of the transmission spectrum 8 of the finesse variable Fabry-Perot interferometer is
When performing control to match the frequency that gives
The above control is performed at , and then the above control is performed at relatively large finesse 12 .

Although FIG. 7B shows the light receiver 7 receiving light emitted in the same direction as the incident light of the variable finesse Fabry-Perot interferometer, the present invention is not limited to this drawing. The present invention is not limited to this, and the configuration may be such that the light emitted in the opposite direction to the incident light is received via, for example, an optical circulator, and in this case as well, the received light will be referred to as transmitted light of the interferometer.

Function Now, in FIG. 1(a), if the arbitrary magnetic field applied to the Faraday rotator 4 is O, then the Q value when this interferometer is viewed as a resonator is the polarizer 3 and the Faraday rotator. 4 and the transmittance of the optical medium interposed between these and the reflecting mirror 1.degree. 2, and the resonator optical path length, and its value is a constant value determined by the configuration conditions of the apparatus. When a non-zero magnetic field is applied to the Faraday rotator 4, the plane of polarization of light transmitted through the Faraday rotator 4 rotates in accordance with this magnetic field, so the amount of light removed in the polarizer 3 in a direction different from the resonator optical axis increases. The result is different from that when the applied magnetic field is 0, and the apparent transmittance of the polarizer 3 has changed accordingly. When the transmittance of the polarizer 3 changes, the Q value of this resonator changes, which equivalently means that the reflectance of the reflecting mirror 1 or 2 changes, and the finesse of the transmission spectrum changes. . Furthermore, Faraday rotator 4
The reason why the finesse is changed by changing the applied magnetic field is to obtain the desired finesse without mechanical movement.
This is also to enable quick changes in finesse.

In the oscillation frequency stabilization method shown in FIGS. 1(b) to 1(d), the oscillation frequency of the semiconductor laser is controlled to match the peak intensity of the transmission spectrum or the frequency that gives a predetermined intensity. The control method shown in Fig. 12 in which the oscillation frequency is controlled to match the frequency that provides the peak intensity using synchronous detection, or the control method shown in Fig. 10 that uses the reference power source to control the oscillation frequency to match the frequency that provides the predetermined intensity. This is because the control method is assumed. Regardless of the control method, by stabilizing the frequency with a relatively small finesse using a variable finesse Fabry-Perot interferometer, a wide pull-in width on the frequency axis is achieved, and the frequency is stabilized with a relatively large finesse. By performing stabilization, high frequency stability can be obtained.

Example Embodiments of the present invention will be described below based on the drawings.

Note that the same reference numerals indicate the same members throughout the drawings.

FIG. 2 is a configuration diagram of a variable finesse Fabry-Perot interferometer showing an embodiment of the present invention, in which interference light is extracted in the same direction as the direction of incident light. 21.27
are high reflectivity mirrors arranged to face each other so as to form a resonator structure, and a polarization splitting prism 22 and a Faraday rotator 29 are provided in this order in the propagation direction of the incident light between these mirrors. Reference numeral 26 denotes an electromagnet for applying an arbitrary magnetic field in the direction of the transmission optical axis of the Faraday rotator 29, and is constructed by winding a coil around the Faraday rotator 29, for example. The polarization separation prism 22 is configured by interposing a polarization separation film 25 between the slopes of triangular prisms 23 and 24, and separates the polarization component having a plane of polarization parallel to the plane of the paper out of the light propagating in the optical axis direction of the resonator. The P-wave component (to the polarization separation film 25) is transmitted in the direction of the optical axis of the resonator, and the polarization component (S-wave component to the polarization separation film 25) having a plane of polarization perpendicular to the plane of the paper is transmitted to the resonator light. It functions to reflect in a direction different from the axial direction (dotted line direction in the figure).The light incident through the high reflectivity mirror 21 is usually light from the LD, and the light from the LD is almost linearly polarized light. Therefore, by making the plane of polarization parallel to the paper plane, the substantial transmittance of the polarization separation prism 22 in the resonator optical axis direction can be increased.
As a result, the maximum finesse in the finesse variable range of this interferometer can be increased.

When the plane of polarization of the incident light is set as described above, when the electromagnet 26 is energized to impart optical rotation to the Faraday rotator 29 made of YIG (yttrium-iron-garnet), etc., the Faraday rotator 29 is rotated. Since the plane of polarization of the light that is transmitted back and forth is rotated appropriately, the polarization separation prism 22
In this case, light is reflected and separated in a direction different from the resonator optical axis depending on the optical rotation angle. As a result, the Q value in the resonator structure decreases, which means that the reflectivity of the high reflectivity mirror decreases isometrically, and the finesse decreases.

The amount of change in finesse at this time is the Faraday rotator 29
Since the finesse is determined according to the angle of optical rotation of the electromagnet 26, the required finesse can be set by controlling the current value of the electromagnet 26.

FIG. 3 shows a configuration in which the high reflectance mirror 27 in the configuration shown in FIG. 2 is replaced with a total reflection mirror 28. Even if the shape of the reflecting mirror is different between the incident side and the opposite side of the resonator structure, the transmitted light of this interferometer that is reflected by the total reflection mirror 28 and taken out via the high reflectance mirror 21 is , the transmission spectrum has a transmission spectrum in which peaks appear periodically on the frequency axis, and the finesse of the spectrum peak can be varied by the current flowing through the electromagnet 26, as in the embodiment shown in FIG.

FIG. 4 shows an example in which each optical component of the interferometer shown in FIG. 2 is integrally constructed. In this example, a polarization separation prism 31 with a polarization separation film 34 interposed between triangular prisms 32 and 33 and a Faraday rotator 35 are bonded together, and high reflectance mirrors 36 and 37 are attached to both end faces of these, respectively. is formed by, for example, vapor deposition, and an electromagnet 38 is arranged around a Faraday rotator 35 to constitute the entire structure. According to such an integrated configuration, it is easy to keep the resonator optical path length constant with high precision, and it is also necessary to take into account changes in the refractive index due to changes in temperature of the optical medium (air, etc.) between each optical component. , so high frequency stability can be easily obtained. Although this embodiment uses a resonator structure using a pair of high-reflectance mirrors, either one may be used as a total reflection mirror to obtain the same function as in FIG. 3. By doing this, even if the loss due to absorption in the Faraday rotator cannot be ignored, there is no possibility that the maximum finesse in the finesse variable range will decrease due to the absorption.

In the examples shown in FIGS. 2 to 4, in order to increase the maximum finesse in the finesse variable range, the plane of polarization of the incident light is made to match the plane of polarization of the P-wave component of the polarization separation film. FIG. 5 shows a configuration that does not require setting this polarization plane. In addition to the configuration shown in FIG. 3, here, a Faraday rotator 41 and a total reflection mirror 43 are arranged on the optical axis of reflection of the incident light of the polarization separation film 25, and an arbitrary magnetic field is applied to the Faraday rotator 41. An electromagnet 42 is provided. According to this configuration, since the transmitted light and the reflected light of the polarization separation film 25 mutually compensate, it is possible to obtain stable finesse regardless of the polarization plane of the interferometer incident light.

By eliminating the polarization plane dependence of finesse, it is no longer necessary to use a polarization constant fiber as an optical fiber for optically connecting this interferometer to a light source. Note that, in order to obtain a stable transmission spectrum waveform, it is desirable that the resonator length on the transmission optical axis of the polarization separation film 25 and the resonator length on the reflection optical axis match.

Figure 6 is a variable finesse Fabry-Perot interferometer (Figure 3)
FIG. 2 is a block diagram of an LD oscillation frequency stabilizing device configured using the following. Three L's that should stabilize the oscillation frequency
The output lights of D51, 52, and 53 are mixed by the star coupler 54, and a part of the mixed light is sent to the optical circulator 55.
The light is input to a variable finesse Fabry-Perot interferometer 56 through the finesse variable Fabry-Perot interferometer 56. The output light of the Fabry-Perot interferometer 56 is incident on a light receiver 57 via an optical circulator 55.

58, 59, and 60 are control circuits for controlling the bias current values of LDs 51 and 52, 53, respectively. In this embodiment, in order to stabilize the frequency of three LDs at the same time,
Each control circuit 58, 59, 60 is connected to a low frequency oscillator 61, 62, 63 that oscillates at a low frequency rl, r, F3 that does not affect signal transmission, and each LD 51, 52, 53 It is designed to be frequency modulated by frequency. Therefore, after making the oscillation frequency of each LD approximately match the frequency that gives the peak of the transmission spectrum of the Fabry-Perot interferometer 56, the change signal of the received light level for each LD based on frequency modulation is transmitted to the low frequency oscillator 61, By performing synchronous detection with lock-in amplifiers 64 and 65.66 connected to 62.63, respectively,
Fluctuations in the oscillation frequency of each LD can be extracted, and based on this, each detection output can be fed back to the bias current of each LD to stabilize each oscillation frequency.

FIG. 7 is a diagram for explaining the procedure of the stabilization method depending on the finesse of the Fabry-Perot interferometer 56. Figure (a) shows the case when the Fabry-Perot interferometer 56 is energized and the finesse is set small. Prior to feedback control of the LD bias current, first, the transmission spectrum 7 of the Fabry-Perot interferometer 56 in this state is shown.
The bias current values are set so that the oscillation frequencies of the LDs 51, 52, and 53 approximately match the frequencies F, F, F, which give a peak of 1, respectively. Next, frequency discrimination of each LD is performed using the detection output cuffs 2, 73.74 of the lock-in amplifiers 64, 65.66, which correspond to the first-order differential curve of the spectrum peak 71, and the oscillation frequency of each LD is set to the corresponding set frequency. Stabilization control is performed to match (F,, F2°F3). At this time, since the finesse of the spectral peak is small, the slope of the discrimination characteristic is relatively small, and therefore the frequency width in which the oscillation frequency of each LD can be drawn can be widened. When the frequency stabilization at a small finesse reaches a steady state, the current flowing through the Fabry-Perot interferometer 56 is decreased to increase the finesse of the transmission spectrum 75 as shown in (1) in the same figure, thereby increasing the detection output ( Frequency discrimination characteristics) 76.
Frequency stability is further increased by increasing the slope of 77.78. Note that although the frequency pull-in width at this time is relatively small, it does not pose a problem because the feedback control described using FIG.

The above stabilization procedure is effective for initial control when stabilizing the oscillation frequency of each LD at equal intervals on the frequency axis, but
Even when the oscillation frequency of one of the LDs fluctuates by the amount of pull-in width due to an unexpected cause, the above procedure may be performed again.

Effects of the Invention As detailed above, according to the present invention, when stabilizing the oscillation frequency of a semiconductor laser, it is possible to maintain high frequency stability and widen the pull-in width on the frequency axis when frequency stabilization is performed. This has the effect of making it possible.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the principle of the present invention, FIGS. 2 to 7 are diagrams showing embodiments of the present invention, and FIGS. 8 to 14 are diagrams showing the prior art. 1.2... Reflector, 3... Polarizer, 4.29, 35.41... Faraday rotator, 6.5
1,52.53...Semiconductor laser (LD), 7.57
... Light receiver, 21.27, 36.37 ... High reflectance mirror, 28.
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Claims (2)

[Claims] (1) Consists of a pair of reflecting mirrors (1, 2) and a Q-value variable means inserted between them, and the Q-value variable means includes a polarizer (3) and a Faraday rotator (
4) and means (5) for applying an arbitrary magnetic field to the Faraday rotator (4). (2) The light from the semiconductor laser (6) is transmitted through the finesse variable Fabry-Perot interferometer according to claim 1, and the light receiver (
7), and control the oscillation frequency of the semiconductor laser (6) to match the frequency that gives the peak intensity (9) or predetermined intensity (10) of the transmission spectrum (8) of the finesse variable Fabry-Perot interferometer. A method for stabilizing the oscillation frequency of a semiconductor laser, characterized in that the above control is first performed with a relatively small finesse (11), and then the above control is performed with a relatively large finesse (12).