JPH0697557A - Laser frequency sweeping equipment - Google Patents

Abstract

PURPOSE:To enable frequency sweeping of continuous phase over a wide frequency range, while keeping narrow line width. CONSTITUTION:A laser equipment 1 is equipped with a frequency setting means capable of changing the frequency of output light without changing the intensity of the output light. In order to constrict the spectrum line width of the output light of the laser equipment 1 and change continuously the frequency of the output light, a frequency variable frequency discriminator 2 driven by a driving equipment 3 is installed, and optical self-injection synchronization is generated. In order to keep the state of optical self-injection synchronization state, a control equipment 5 is installed, which controls a frequency setting means while referring the output of a light receiving equipment 4 for converting the intensity of the output light of the frequency variable frequency discriminator 2 to an electric signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser frequency sweeping device, which is used as a light source in the fields of optical communication, light wave measurement and the like, and which is capable of sweeping a wide frequency band while maintaining a narrow spectral line width.

[0002]

2. Description of the Related Art As a semiconductor laser device whose oscillation wavelength is variable and whose oscillation spectrum line width is narrowed, there is proposed a semiconductor laser device disclosed in, for example, Japanese Patent Laid-Open No. 2-1566691. In FIG.
A cross-sectional configuration diagram of a semiconductor laser device having a variable oscillation wavelength disclosed in Japanese Patent No. 91 is shown. The semiconductor laser device of FIG.
The active region 11, the phase control region 12 which is continuous with the active region 11 with the boundary 18 in between, and a reflecting mirror (reflection means) for returning the light emitted from the phase control region 12 side to the phase control region 12 again. 20 and a lens 21 for allowing the light emitted from the phase control region 12 to enter the reflecting mirror 20 and coupling the light reflected by the reflecting mirror 20 to the phase controlling region 12 again. An active layer 15 is formed on the active region 11, and a diffraction grating 14 is formed on the entire active layer 15. In addition, the active region 1
1 is an electrode 16 for injecting current into the active layer 15.
Is provided, and the emission end face 19a of the active region 11 is AR-coated. In the phase control region 12, an optical waveguide layer 13 is formed continuously with the active layer 15, and an electrode 17 for injecting a current into the optical waveguide layer 13 is provided. Phase control area 12
An AR coating is applied to the light emitting end face 19b of the.

Next, the operation of this semiconductor laser device will be described. By injecting the current Id into the electrode 16 of the active region 11, the gain of the active layer 15 increases, and when the loss of the resonator is exceeded, laser oscillation occurs. Here, the end face 19
Since the AR coating is applied to a and 19b, the resonator is composed of the diffraction grating 14 and the reflecting mirror 20. Because they act only as a reflecting mirror to light having a wavelength in this case the diffraction grating 14 is the Bragg wavelength lambda _g near the single-mode oscillation occurs at the Bragg wavelength lambda _g wavelength in the vicinity. Where n _p (I _p )
Is the refractive index of the optical waveguide layer 13 depending on the injection current I _p , L
_{When p} is the length of the optical waveguide layer 13, n _e is the refractive index between the end face 19b and the reflecting mirror 20, and L _e is the distance between the end face 19b and the reflecting mirror 20, the following equation is satisfied: Oscillates at the Bragg wavelength λ _g . N + 3/4 = [2n _p (I _p ) L _p + 2n _e Le _e ] / λ _g (1) Here, N is an integer.

Therefore, if the current I _p injected into the phase control region 12 is set so as to satisfy the expression (1), the oscillation wavelength of the laser can be set to the Bragg wavelength λ _g .
Further, by shifting the value of the current I _p from this set value, the oscillation wavelength of the laser can be shifted from the Bragg wavelength λ _g . That is, the oscillation wavelength of the laser can be changed by the current I _p injected into the phase control region 12.
Further, in the conventional semiconductor laser device, since the reflecting mirror 20 is set outside, the resonator length can be increased and the Q value of the resonator can be increased. Therefore, the oscillation spectrum line width can be further narrowed.

[0005]

As described above, in the conventional semiconductor laser device, the oscillation wavelength of the laser can be varied by the current I _p injected into the phase control region 12, and the oscillation spectral line width is reduced. The semiconductor laser device is narrowed. However, when the laser frequency is swept using the conventional semiconductor laser device, the current I _p injected into the phase control region 12 is changed, but the oscillation threshold of the Bragg wavelength oscillation mode is changed by the change of the current I _p. Since the value gain increases and the oscillation threshold gain of the Fabry-Perot mode formed by the active layer end face 19a and the reflecting mirror 20 decreases, the phenomenon that the oscillation mode changes to the Fabry-Perot mode occurs. Therefore, the conventional semiconductor laser device is
There are problems that the frequency sweep range is narrow in continuous phase, continuous oscillation is not possible because the oscillation modes are scattered, and the frequency is not stable. An object of the present invention is to solve the above-mentioned problems and to provide a laser frequency sweeping device capable of sweeping a frequency in a continuous phase over a wide band while maintaining a narrow spectral line width.

[0006]

In order to solve the above-mentioned problems, the laser frequency sweeping device of the present invention comprises frequency setting means capable of varying the frequency of the output light without changing the intensity of the output light in the laser device. The laser device is provided with a frequency variable frequency discriminator for narrowing the spectral line width of the output light of the laser device and for continuously changing the frequency of the output light to generate optical self-injection locking. And controlling the frequency setting means by referring to the output of the photodetector which converts the intensity of the output light of the frequency variable frequency discriminator into an electric signal by providing a controller for maintaining the optical self-injection locked state. I am trying to do it.

That is, a laser device provided with a frequency setting means capable of varying the frequency of the output light without changing the intensity of the output light, and receiving light from the laser device to generate optical self-injection locking of a specific frequency. A frequency variable frequency discriminator for generating light, a drive device for driving the frequency variable frequency discriminator to continuously sweep the specific frequency, and an output light intensity of the frequency variable frequency discriminator A light receiver for converting into a signal and a controller for controlling the frequency setting means so as to maintain the state of optical self-injection locking by referring to the output of the light receiver.

[0008]

In the semiconductor laser, when the oscillated laser beam is incident again, the oscillating frequency is pulled to the frequency of the incident laser beam and the oscillation line width is narrowed. This phenomenon is called optical self-injection locking. In the present invention, a laser beam that resonates only at a specific frequency is injected into a semiconductor laser, and
A frequency variable frequency discriminator capable of varying the specific frequency is used. During the oscillation frequency sweep, the drive unit drives the variable frequency frequency discriminator to change the frequency discriminated by the variable frequency frequency discriminator. At this time, if a certain amount of difference occurs between the specific frequency to be discriminated and the oscillation frequency of the semiconductor laser determined by the injection current when there is no return light from the frequency variable frequency discriminator, optical self-injection locking Therefore, the intensity of the output light of the frequency variable frequency discriminator is converted into an electric signal by the light receiver, and the frequency setting means is controlled by the controller while referring to the output of the light receiver. As a result, the state of optical self-injection locking is maintained.

That is, in the device of the present invention, it is possible to suppress the occurrence of the mode change caused by the change in the gain profile between the modes occurring during the frequency sweep, which has occurred in the prior art. It will be possible. Further, since the laser device can change the frequency of the output light by the frequency setting means without changing the intensity of the output light,
During the frequency sweep, the state of optical self-injection locking is stably maintained over a wide optical frequency range, enabling wideband frequency sweep.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing a first embodiment of the present invention. The laser frequency sweeping device of the first embodiment is a DBR
(Distributed Bragg Reflect
tor: distributed reflection type laser (laser device) 1, the DB
Resonance frequency variable etalon (frequency variable frequency discriminator) 2 arranged at a position for receiving the output light of the R laser 1, and a piezo element 3a provided on a mirror forming the etalon 2.
A piezo controller (driving device) 3, including a light receiver 4, which is arranged at a position for receiving the output light of the etalon 2, and the light receiver 4
And a control device 5 for receiving the output of
Is a phase detector 5a that receives the output of the photodetector 4, an injection current source 5b that receives the output of the phase detector 5a, and a modulation signal source that applies a modulation signal to the injection current source 5b and the phase detector 5a. 5c.

D used as the laser device of the first embodiment
The BR laser 1 has a structure as shown in FIG. 3, for example. The DBR laser of FIG. 3 is formed of an active region where the output power can be changed, a phase control region where the phase of light inside the DBR laser is adjusted, and a DBR region where the oscillation frequency can be changed without changing the output power. ing. By independently injecting current into each region, the output power, phase, and oscillation frequency can be adjusted independently. The DBR area corresponds to the frequency setting means. In FIG. 1, the injection current source to the active region and the phase control region is omitted.

The operation of the first embodiment will be described. The light emitted from the DBR laser 1 resonates with the etalon 2, one light returns to the DBR laser 1 again, and the other light passes through the etalon 2 and is received by the light receiver 4. Etalon 2
Is a resonator in which two mirrors are opposed to each other, and the frequency (resonance frequency) of light reflected and transmitted is determined only by the distance between the mirrors. In a semiconductor laser, when the oscillated laser beam is incident on the semiconductor laser again, the oscillation frequency is pulled to the frequency of the incident laser beam and the oscillation line width is narrowed (this phenomenon is called optical self-injection locking). It has the property. Therefore, the oscillation frequency of the DBR laser 1 is stabilized at the resonance frequency of the etalon 2, and the oscillation line width is narrowed.

When optical self-injection locking is occurring, D
When the injection current to the DBR region of the BR laser 1 is swept, the amount of reflected light and the amount of transmitted light from the etalon 2 obtained by the light receiver 4 are as shown in FIG. The output light quantity when the return light for generating the light self-injection locking is not applied is as shown in FIG. In FIG. 5, a steep peak is seen in the injection current value corresponding to the resonance frequency of the etalon 2, but in the case of FIG. 4 in which optical self-injection locking occurs, the shape is rectangular. This rectangular portion (A portion) means that the amount of reflected light and the amount of transmitted light from the etalon 2 do not change even if the injection current to the DBR region is changed. That is, D
This means that the oscillation frequency of the BR laser 1 is stabilized. The rectangular portion is called a rocking range.

The oscillation frequency is swept by continuously changing the distance between the mirrors of the etalon 2 by the piezo controller 3 while maintaining the state where the optical self-injection locking is generated.
When the mirror spacing of the etalon 2 changes, the resonance frequency changes. At this time, if the oscillation frequency of the semiconductor laser, which is determined by the injection current to the DBR region when there is no returning light, deviates from the locking range, the optical self-injection locked state does not occur. Therefore, at the time of frequency sweep, it is necessary to make the oscillation frequency follow the changing resonance frequency within the rocking range by controlling the injection current to the DBR region. This frequency tracking control is performed by controlling the injection current into the DBR region of the DBR laser 1 while referring to the amount of transmitted light from the etalon 2 detected by the light receiver 4.

That is, the injection current from the injection current source 5b is modulated by the signal from the modulation signal source 5c, and the DBR laser 1
Is injected into the DBR region. The DBR laser 1 emits laser light having a frequency corresponding to the modulated injection current and the frequency of the return light. Of the emitted laser light, the light that resonates with the etalon 2 and that has passed through the etalon 2 is received by the light receiver 4. The light receiver 4 outputs a signal corresponding to the power of the received light to the phase detector 5a. The phase detector 5a uses the output of the light receiver 4 to transmit the amount of transmitted light of the etalon 2 corresponding to the maximum value of the modulated injection current and the etalon 2 corresponding to the minimum value thereof.
Of the transmitted light is detected and output to the injection current source 5b.

The output of the phase detector 5a is as shown in FIG. 7, and corresponds to the differential waveform of the amount of light reflected and transmitted from the etalon 2 during the optical self-injection locking shown in FIG.
The portion where the output is zero (the portion B) corresponds to the rectangular portion (the portion A) in FIG. When the minimum value side of the modulated injection current deviates from the rectangular area of FIG. 4, the output of the phase detector 5a swings to the positive side as shown in FIG. 7, and the maximum value side of the rectangular area of FIG. When it comes off, the phase detector 5
The output of a swings to the negative side as shown in FIG.

The rectangular area changes as shown in FIG. 6 in accordance with the change in resonance frequency. That is, the etalon 2 without changing the injection current (eg, fixed at ia)
If the distance between the mirrors is changed to La, Lb, and Lc (resonance frequency is changed), the rectangular portion A is eventually deviated from (a state of optical self-injection locking). The injection current source 5b receives the output of the phase detector 5a and increases or decreases the injection current into the DBR region so that the output becomes zero. By controlling in this way, even if the distance between the mirrors of the etalon 2 is changed (resonance frequency is changed), the etalon 2 does not deviate from the rectangular region (state of optical self-injection locking). Such an electrical control method performed by using the control device 5 including the phase detector 5a, the injection current source 5b, and the modulation signal source 5c is an example of a method for controlling the injection current applicable to the present invention.

FIG. 2 is a schematic configuration diagram showing a second embodiment of the present invention. The laser frequency sweeping device of the second embodiment is substantially a resonant frequency variable device arranged at a position for receiving the DBR laser 1 and the output light of the DBR laser 1 of the laser frequency sweeping device of the first embodiment. A second piezo controller 7 including a corner mirror 6 for changing the optical path length and a second piezo element 7d provided on the corner mirror 6 is added between the etalon 2 and the etalon 2. The corner mirror 6 and the second piezo controller 7 constitute means for varying the optical path length from the DBR laser 1 to the etalon 2, and the second piezo controller 7 is a microcomputer 7a and an applied voltage described later. V _e - includes a memory 7b which stores applied voltage V _c table and applied voltage source 7c and the second piezoelectric element 7d.

The operation of the second embodiment will be described. The light emitted from the DBR laser 1 is reflected by the corner mirror 6 and enters the etalon 2. The incident light resonates with the etalon 2 and one light returns to the DBR laser 1 via the corner mirror 6, while the other light resonating with the etalon 2 passes through the etalon 2 and is received by the light receiver 4. Below, D
The oscillation frequency of the BR laser 1 is stabilized at the resonance frequency of the etalon 2 and the oscillation line width is narrowed. The sweeping of the laser frequency is the first while maintaining the state where the optical self-injection locking is generated. The piezo controller 3 continuously changes the distance between the mirrors of the etalon 2, and the maintenance of the state in which the optical self-injection locking is generated refers to the amount of transmitted light from the etalon 2 detected by the light receiver 4. While DBR laser 1
The same as in the first embodiment is performed by controlling the injection current into the DBR region.

The functions of the corner mirror 6 and the second piezo controller 7 will now be described. When the mirror interval of the etalon 2 is changed by the piezo element 3a in order to sweep the oscillation frequency of the DBR laser 1, the feedback laser light to the DBR laser 1 resonated by the etalon 2 and the DBR laser 1
There is a phase shift with the internal laser light. The phase shift causes the shape of the transmission spectrum of the etalon 2 to change. If the shape of the transmission spectrum of the etalon 2 changes, the state of optical self-injection locking becomes unstable. Therefore, the corner mirror 6 and the second piezo controller 7 change the optical path length from the emission end face of the DBR laser 1 to the incident end face of the etalon 2 so as to prevent phase shift.

The above correction is performed when a voltage V _e is applied to the piezo element 3a for changing the mirror spacing of the etalon 2.
The voltage V _c applied to the piezo element 7d provided in the corner mirror 6 necessary for correcting the phase shift is measured in advance, and the voltage V _e applied to the piezo element 3a for changing the mirror spacing of the etalon 2 is measured. the reference to the applied voltage V _c table, corner mirror 6 to the applied voltage source 7c - in response to the second microcomputer 7a provided to the piezo controller within 7 applied voltage V _e which is stored in the memory 7b The voltage V _c is applied to the piezo element 7d.

Although the DBR laser is used as the laser device in the first and second embodiments, a laser device or the like which can change the frequency of the output light without changing the intensity of the output light by changing the temperature may be used. Good.

In the device of the present invention, it is possible to suppress the occurrence of the mode change caused by the change of the gain profile between the modes which occurs in the frequency sweep, which occurs in the prior art, and therefore the frequency sweep can be performed continuously in phase. Become.
Further, since the laser device can change the frequency of the output light without changing the intensity of the output light by the frequency setting means, the state of optical self-injection locking is stably maintained in a wide optical frequency range during the frequency sweep, and the broadband Frequency sweep is possible. Specifically, DFB (Distributed Fe)
If an edback (distributed feedback type) laser is used,
Due to the change in the intensity of the output light caused by the injection current control performed during the frequency sweep, the frequency sweep width is at most about several tens of GHz, whereas when the DBR laser is used, the injection current control is performed. Since the intensity of the output light does not change even if it goes, it corresponds to the range of the refractive index change of the DBR laser of several 100 G.
A frequency sweep of about Hz becomes possible.

According to the present invention, the frequency can be continuously swept over a wide band while maintaining a narrow spectral line width even if the phase shift is not corrected as in the first embodiment. By correcting the phase shift as in the embodiment described above, the stability of the optical self-injection locked state at each frequency during the frequency sweep is further improved, and the fluctuation of the oscillation line width and the output power during the frequency sweep is zero. become.

[0025]

According to the laser frequency sweeping device of the present invention, the laser device is provided with a frequency setting means capable of varying the frequency of the output light without changing the intensity of the output light. In order to narrow the spectral line width,
And a frequency variable frequency discriminator for continuously changing the frequency of the output light to generate optical self-injection locking, and a controller for maintaining the state of the optical self-injection locking to provide the frequency. Since the frequency setting means is controlled by referring to the output of the photodetector that converts the intensity of the output light of the variable frequency discriminator into an electric signal, the phase continuity is maintained over a wide band while maintaining a narrow spectral line width. We have obtained a laser frequency sweeper that can sweep the frequency at.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram showing a second embodiment of the present invention.

FIG. 3 is a diagram showing a structure of a DBR laser.

FIG. 4 is a diagram showing a transmission spectrum of an etalon when optical self-injection locking occurs.

FIG. 5 is a diagram showing a transmission spectrum of an etalon when no return light for generating optical self-injection locking is applied.

FIG. 6 is a diagram for explaining that a region in a state of optical self-injection locking changes with a change in resonance frequency;
(A) is a region where the etalon mirror is La, (b) is a region where the etalon mirror is Lb, and (c) is a region in the state of optical self-injection locking when the etalon mirror period is Lc and the injection current. The figure which shows each relationship with ia.

FIG. 7 is a diagram showing an output of a phase detector.

FIG. 8 is a sectional configuration diagram of a conventional semiconductor laser device.

[Explanation of symbols]

 1 Laser Device (DBR Laser) 2 Frequency Variable Frequency Discriminator (Etalon) 3 Drive Device (Piezo Controller) 4 Light Receiver 5 Control Device 6 Corner Mirror 7 Second Piezo Controller

Claims (1)

[Claims] 1. A laser device (1) comprising frequency setting means capable of varying the frequency of output light without changing the intensity of output light.
A frequency variable frequency discriminator (2) for receiving light from the laser device to generate light for generating optical self-injection locking of a specific frequency; and driving the frequency variable frequency discriminator to generate the specific frequency. Drive device for continuously sweeping (3)
A photoreceiver (4) for converting the intensity of the output light of the variable frequency discriminator into an electric signal, and the frequency setting so as to maintain the state of optical self-injection locking with reference to the output of the photoreceiver. A laser frequency sweeping device comprising a controller (5) for controlling the means.