JPH10107354A - Spectral linewidth control device for semiconductor laser beam source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost spectral linewidth control device, facilitating optical system composition, without increasing adjusting steps, within an outer resonator type semiconductor laser beam source. SOLUTION: An outer resonator type laser beam source is composed of a semiconductor laser oscillator by a semiconductor laser 11 having a non- reflecting mirror 12, a reflecting mirror 6 comprising a resonator together with the oscillator, and a laser drive circuit 14. Furthermore, the laser beam source is provided with a random noise producer 3, convoluting an output current with a laser driving current, a switch 15 convoluting and disconnecting the noise current with and from the laser drive current, as well as a controller 16 controlling the switch transfer operation and the noise current of the random noise current of the random noise producer 3. Through these procedures, the noise current by the random noise producer 3 is convoluted with the laser driving current at an arbitrary amplitude through the intermediary of the switch 15 by the controller 16. In such a constitution, the fluctuation in the laser drive current by the noise current becomes the fluctuation in optical frequency, thereby equivalently widening the spectral linewidth.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spectral line width controller for an external resonator type semiconductor laser light source.

[0002]

2. Description of the Related Art There is an external cavity type semiconductor laser light source as a widely variable wavelength light source (hereinafter, a semiconductor laser is abbreviated as LD). The spectral line width of such an external resonator type LD light source has been increased to 100 kHz with the increase in the length of the resonator.
The following has been obtained. In contrast, the spectral line width of a normal LD alone is on the order of 10 MHz.

However, in the measurement using the external cavity type LD light source as described above, the good coherence due to the narrow spectral line width (hereinafter referred to as coherency) is conversely.
In some cases, the interference noise causes a measurement error. Usually, a reflection point exists in the measurement system, and interference occurs between the measurement light and the reflected light. The interference light intensity depends on the optical path difference between the two lights, and the optical path difference always fluctuates, so that the measurement light intensity fluctuates as interference noise. In addition, since the optical path difference fluctuation involves a slow change due to a temperature change or the like, it may not be reduced even by the averaging process depending on the measurement conditions.

By the way, when the output light from the same light source is branched into two light beams, passing through different optical path lengths, and then multiplexed again, the maximum optical path length difference at which interference fringes can be obtained is determined by the coherent distance (hereinafter referred to as the coherence distance). , Called the coherence length). The coherence length D is given by the following equation (1), where the spectral line width is Δν. D = c / Δν (1) where c is the speed of light.

From the equation (1), for example, when the spectral line width Δν is 100 kHz, the coherence length D capable of interfering is about 3 km, and when it is 10 MHz, it is 30 m.
That is, depending on the optical path length conditions of the measurement system, interference noise that has not been observed when measuring with the LD alone in the related art may become significant due to the use of the external resonator type LD. In addition, recently, LDs with a narrow spectral line width have been reported, even when used alone, so that interference noise appears even when used.

As a method of reducing the interference noise, there is a method of applying a stress to the output optical fiber of the light source to change the polarization state of the output light. However, in this method, since the optical frequency does not change, only the same effect as the above-mentioned averaging process can be expected. Therefore, a method of controlling the spectral line width by changing the optical frequency of the LD light source has been conventionally used. As such a spectral line width control device, for example, there is one as disclosed in JP-A-6-37382.

Next, FIG. 3 shows an example of the configuration of a conventional apparatus for controlling the spectral line width of an LD light source. Note that FIG.
The spectral line width control device shown in FIG.
Since this is the same as that shown in FIG. 2 of Japanese Patent Publication No. 382, the detailed description of the operation is omitted, and only the configuration will be briefly described below. That is, in FIG.
, And a laser oscillation is obtained by having the gain medium 1 in the resonator. Here, assuming that the resonator length is L, the optical frequency f is given by the following equation (2). f = Nc / (2L) (2) where N is an integer.

From this equation (2), the optical frequency change Δf with respect to the resonator length change ΔL is given by the following equation (3). Δf = −fΔL / L (3) From the equation (3), for example, the optical frequency f is set to 200 TH
z, the cavity length L is 50 mm, and the cavity length change ΔL is a vibration with an amplitude of ± 2.5 nm.
f is about ± 10 MHz. Also, the resonator length change ΔL
If the vibration of is random, the optical frequency change Δ
f is equivalent to the spectral line width Δν equivalently,
The conditions are almost the same as those of a single unit. As shown in FIG. 3, the minute displacement actuator 2 is attached to the reflecting mirror 6, and the minute displacement actuator 2 applies random vibration in the optical axis direction to the reflecting mirror 6 by a signal from the random noise generator 3. give. The resonator length fluctuates with the vibration of the reflecting mirror 6, and the optical frequency fluctuates.

Next, FIG. 4 shows an example of the configuration of another conventional spectral line width control device for an LD light source. The spectral line width control device shown in FIG.
Since the operation is the same as that shown in FIG. 3 of Japanese Patent No. 7382, detailed description of its operation is omitted, and only the configuration will be briefly described below. That is, in FIG. 4, a resonator is formed by the reflecting mirror 5 and the reflecting mirror 6, and the laser oscillation is obtained by having the gain medium 1 in the resonator. By driving the phase modulator 4 in the resonator by the random noise generator 3, the refractive index of the resonator fluctuates.
The change in the refractive index in the resonator changes the length of the resonator, and the optical frequency changes. In addition, as a phase modulator, there is also a phase modulator integrated in an LD.

[0010]

However, first, in the conventional example shown in FIG. 3, a micro displacement actuator 2 is newly required to generate vibration. Further, since the micro-displacement actuator 2 is attached to the reflecting mirror 6 of the optical system, the man-hour for assembling and adjusting the light source unit increases. The frequency band of the random noise generator 3 is several tens MH.
frequency band of mechanical vibration is several tens of k
Since the frequency is up to Hz, there is also a problem that the effect of reducing interference noise due to optical frequency fluctuation is reduced under high-speed measurement conditions.

Further, in the conventional example shown in FIG. 4, since the phase modulator 4 is set in the resonator, the optical adjustment becomes complicated. In that regard, the use of a phase modulator integrated LD can suppress an increase in the number of adjustment steps, but such an integrated LD is
There is a problem that the manufacturing process is complicated and the cost is higher than that of a normal LD. In addition, in the case of a single LD having a narrow spectral line width, there is a problem that the LD must be a phase modulator integrated type.

Accordingly, the present invention provides an external resonator type LD.
It is an object of the present invention to provide an inexpensive spectral line width controller in which a light source has a simple optical system configuration, does not increase the number of adjustment steps, and is inexpensive.

[0013]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the invention according to claim 1 has an output side resonator mirror on one end face, and one end face having the output side resonator mirror and the opposite side. L that emits laser light from both sides of the other end surface of the anti-reflection film or the like
D oscillation means, a reflecting mirror arranged on the optical axis of the laser light on the other end surface side of the LD oscillation means and forming a resonator by the LD oscillation means, and a laser drive current supplied to the LD oscillation means. And a laser drive circuit for supplying
The D light source includes, for example, an oscillator using a random noise generator or the like, and a drive current variable control unit including, for example, a switch and a control unit that controls the laser drive current by changing the laser drive current based on the output of the oscillator. The configuration of the spectral line width control device for an LD light source is characterized.

Here, as the LD oscillation means, for example,
It is composed of an LD having an anti-reflection film on the other end face opposite to the output side resonator mirror, but may have another configuration. Further, as the drive current variable control means, for example, a switch that superimposes and interrupts an output current from an oscillator such as a random noise generator on the laser drive current, and a control unit that controls the switching operation of the switch and the output current of the oscillator , But may have another configuration.

As described above, according to the first aspect of the present invention, in the LD light source, the driving line variable control means changes the laser driving current based on the output of the oscillator to control the spectral line width. Because it is a device, by variably controlling the laser drive current based on the output of the oscillator,
By changing the optical frequency, it is possible to broaden the spectral line width equivalently. In addition, since only the drive current variable control means for controlling the laser drive current by changing it based on the output of the oscillator is provided, the number of adjustment steps can be increased without modifying the LD light source and keeping the optical system simple. And can be provided at low cost.

According to a second aspect of the present invention, there is provided the spectral line width control device for an LD light source according to the first aspect, wherein the drive current variable control means includes, for example, a signal from the oscillator such as a random noise generator. A switch that performs a switching operation so that an output current due to a noise current or the like can be superimposed on or cut off from the laser driving current; and a switching operation of the switch and an output current (a noise current or the like) of the oscillator (a random noise generator or the like). And a control unit for controlling.

Thus, according to the second aspect of the present invention, the drive current variable control means according to the first aspect of the present invention controls the switching operation of the switch which enables the output current from the oscillator to be superimposed on or cut off from the laser drive current. Since the control section controls the output current of the oscillator by the control section, the control section can superimpose the current on the laser drive current via the switch while controlling the output current of the oscillator. Therefore, the laser drive current can be variably controlled based on the output of the oscillator.

According to a third aspect of the present invention, there is provided the spectral line width control device for an LD light source according to the first or second aspect, wherein the oscillator generates a random noise superimposed on the laser driving current. It features a configuration that is a generator.

As described above, according to the third aspect of the present invention, the random noise generator as the oscillator according to the first or second aspect generates a noise current to be superimposed on the laser drive current. The laser drive current can be varied by superimposing the noise current.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a spectral line width control apparatus for an LD light source according to the present invention will be described below with reference to FIGS. First, FIG. 1 is a block diagram showing a configuration of a spectrum line width control device of an LD light source as an example to which the present invention is applied. However, in FIG. 1, the same reference numerals are given to portions common to the respective portions in FIG. 3 described above, and description thereof will be omitted.

In FIG. 1, reference numeral 11 denotes an LD, 12 denotes an anti-reflection film, 13 denotes a lens, 14 denotes a laser drive circuit, 15 denotes a switch, and 16 denotes a control unit. That is, first, LD
11, an anti-reflection film 12 made of a coating or the like is formed on one end face as shown. Also, LD
11, the other end face opposite to the anti-reflection film 12 and its LD
A resonator is constituted by the above-mentioned reflecting mirror 6 arranged on the optical axis of the anti-reflection film 12 on the side of 11. That is, LD11
The other end face opposite to the non-reflection film 12 is an output-side resonator mirror, and LD oscillation means is constituted by the LD 11 having such an output-side resonator mirror and the anti-reflection film 12 on both end faces. ing. Note that the lens 13 disposed on the optical axis between the LD 11 and the reflecting mirror 6 is a collimator that converts light emitted into the resonator of the LD 11 into parallel light.

Therefore, an external resonator type LD light source is constituted by the LD oscillating means of the LD 11 having the non-reflection film 12 and the output side resonator mirror, the reflection mirror 6, and the collimator lens 13. Further, the laser drive circuit 14 supplies a constant laser drive current to the LD 11. The switch 15 is connected to the laser drive circuit 1
The switching operation is performed to superimpose or cut off the noise current output from the random noise generator 3 as an oscillator on the laser drive current output from the LD 4 to the LD 11. The control unit 16 controls the output current (noise current) of the random noise generator 3 and also controls the switching operation of the switch 15. These switches 15
And the control section 16 constitute a drive current variable control means.

As described above, the spectral line width control device of the external cavity type LD light source is constituted. Therefore, according to the spectral line width control device of the external resonator type LD light source having the above-described configuration, the output value of the noise current from the random noise generator 3 is controlled by the control unit 16. The switch 15 that is switched controls the superimposition on the laser drive current output from the laser drive circuit 14.

Incidentally, in general, the fluctuation of the laser driving current, that is, the injection current to the LD is accompanied by the fluctuation of the light output and the electron density. FIG. 2 is a characteristic diagram showing the relationship between the injection current of the LD light source, the light output, and the electron density. As shown in the characteristic diagram of FIG. 2, until the injection current reaches the threshold current Ith, the optical output is almost zero, and the electron density increases in proportion to the injection current to the threshold electron density Nth. When the current exceeds the threshold current Ith, the injected current is digested as light.
The light output increases proportionately and the electron density increases slightly. This change in electron density is accompanied by a change in refractive index and a change in temperature, resulting in a change in optical frequency.

Here, generally, an InGaAsP-based LD is used.
Is that the change in optical output due to the injection current is about 0.1 mW
/ MA, the optical frequency change due to the injection current is several GHz / mA even after the change amount after Nth becomes dull. However,
In the external resonator type LD, the contribution of the electron density change to the optical frequency change is reduced by the external resonator, so that the optical frequency change due to the injection current is about several hundred MHz / mA. For example, an optical output change corresponding to an optical frequency change of several tens of MHz is about 0.01 mW, and an optical output of 10 mW
It becomes about 0.005 dB in terms of optical output fluctuation at the time. When the optical frequency is further changed, the light intensity fluctuation on the measurement side increases and is observed as noise.In this case, there is no slow fluctuation observed in the interference noise. Can be reduced.

Since the modulation frequency band due to the injection current of the LD is usually about 1 GHz, the fluctuation width of the optical frequency may be regarded as the spectrum line width as it is. Here, the random noise generator 3 is capable of arbitrarily amplifying white noise such as thermal noise or shot noise, for example, noise generated by a zener diode or the like to an output current amplitude of several mA in a band of several tens of MHz or more. Is preferred.
The switch 15 is normally off, and does not control the spectral line width. Therefore, when it is desired to broaden the spectral line width equivalently and degrade the coherency, the control unit 16 switches the switch 15 to the on state,
Further, the output current amplitude of the random noise generator 3 is controlled as needed.

As described above, according to the spectral line width control apparatus for an LD light source according to the embodiment of the present invention, the control unit 1
The optical frequency can be varied by variably controlling the laser drive current based on the output of the random noise generator 3 by switching the switch 15 by 6 so that the spectral line width can be broadened equivalently. Can be.
Moreover, the switch 15 and the control unit 16 for variably controlling the laser drive current based on the output of the random noise generator 3
Is provided, the apparatus for expanding the spectral line width can be provided at low cost without modifying the LD light source, keeping the optical system simple and without increasing the number of adjustment steps. If the reflecting mirror 6 is a diffraction grating, a single-mode laser beam having N as a specific value in the above-described equation (2) can be obtained, and the relative position between the LD 11 and the diffraction grating is appropriately changed. Thus, a spectral line width control device of a wavelength tunable light source capable of changing an optical frequency in a wide range can be configured.

In the above embodiment, the oscillator is a white noise random noise generator. However, the random noise generator may be pink noise or the like. Is used. Needless to say, other components and specific detailed structures can be appropriately changed.

[0029]

As described above, according to the spectral line width control apparatus for an LD light source according to the first aspect of the present invention, the drive current variable control means controls the laser drive current based on the output of the oscillator. Therefore, the optical frequency can be changed by variably controlling the laser drive current based on the output of the oscillator, so that the spectral line width can be equivalently broadened. Moreover, since only the drive current variable control means for changing the laser drive current based on the output of the oscillator is provided, the number of adjustment steps is increased without modifying the LD light source and keeping the optical system simple. No,
There is an effect that an apparatus capable of expanding the spectral line width can be realized at low cost.

According to the apparatus for controlling the spectral line width of an LD light source according to the second aspect of the present invention,
In order to control the oscillator and a switch for superimposing and cutting off the output current from the laser drive current, the current can be superimposed on the laser drive current via the switch while controlling the output current of the oscillator. As described in the first aspect, the laser drive current can be variably controlled based on the output of the oscillator.

According to the spectral line width control apparatus for an LD light source according to the third aspect of the present invention, a random noise generator as an oscillator generates a noise current to be superimposed on the laser drive current. The laser drive current can be varied by superimposing a noise current on the laser drive current.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a spectral line width control device of an LD light source as an example to which the present invention is applied.

FIG. 2 is a characteristic diagram showing a relationship between an injection current of a LD light source, light output, and electron density.

FIG. 3 is a block diagram showing a schematic configuration of a conventional spectral line width control device for an LD light source.

FIG. 4 is a block diagram showing a schematic configuration of another conventional spectrum line width control device for an LD light source.

[Explanation of symbols]

 Reference Signs List 3 random noise generator (oscillator) 6 reflecting mirror 11 semiconductor laser 12 anti-reflection film 13 lens 14 laser driving circuit 15 switch 16 control unit

Claims (3)

[Claims] 1. A semiconductor laser oscillating means having an output-side resonator mirror on one end face, and emitting laser light from both sides of one end face having the output-side resonator mirror and the other end face on the opposite side; A reflecting mirror arranged on the optical axis of the laser light on the other end surface side of the semiconductor laser oscillation means and forming a resonator with the semiconductor laser oscillation means; and a laser for supplying a laser drive current to the semiconductor laser oscillation means A drive circuit, comprising: an oscillator; and a drive current variable control unit that controls the laser drive current by changing the laser drive current based on an output of the oscillator. Width control device. 2. A switch for performing a switching operation so that an output current from the oscillator can be superimposed on or interrupted from the laser driving current, a switching operation of the switch, and an output current of the oscillator. And a control unit for controlling the control.
A spectral line width control device for a semiconductor laser light source according to any one of the preceding claims. 3. The apparatus according to claim 1, wherein said oscillator is a random noise generator for generating a noise current to be superimposed on said laser drive current.