JPH0945983A - Variable wavelength semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a variable wavelength semiconductor laser in which the continuous phase wavelength variation width can be enlarged only through simple adjustment of mechanism without requiring correction for each wavelength. SOLUTION: A laser beam emitted from one edge of a semiconductor laser 1 is branched by an optical demultiplexer 31 and directed partially to a control section 32. The control section 32 controls a resonator or an optical attenuator 30, inserted between the resonator and the semiconductor laser 1, such that the incident laser beam has a constant intensity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength tunable semiconductor laser light source suitable for use as a light source for optical coherent communication or optical coherent measurement.

[0002]

2. Description of the Related Art Generally, a laser exciter (hereinafter referred to as a laser) oscillates at a wavelength at which an oscillation gain has a minimum value. That is, in the external resonator type semiconductor laser, the oscillation gain is a value obtained by subtracting the optical gain of the semiconductor laser from the optical loss of the external resonator, and the optical gain of the semiconductor laser changes gently with the wavelength change, and the external resonance It oscillates at the wavelength where the optical loss of the container is minimized.

By the way, in a resonator composed of two plane mirrors, there are a number of resonance modes in which the reflectance is maximum, that is, the optical loss is minimum. Therefore, when a wavelength filter such as a diffraction grating is used for one of the plane mirrors, oscillation occurs in several or less resonance modes in the wavelength band in which the optical loss determined by the diffraction grating or the like is minimized.

A case will be considered below in which a single mode oscillation external resonator type semiconductor laser using a diffraction grating is used as a light source and the wavelength thereof is variable. In this case, the wavelength changing mechanism that produces the minimum optical loss obtained by the diffraction grating is generally independent of the wavelength changing mechanism of the resonance mode, and two variable mechanisms are required for wavelength tuning. Therefore, the oscillation mode jumps inevitably occur due to the difference between these two changes, and it is difficult to avoid this. That is, many wavelength tunable semiconductor lasers cannot continuously tune their wavelengths because such mode jumps occur.

Therefore, in general, the wavelength that should be tunable by two independent changing mechanisms is continuously tuned by several tens of nm by one tunable mechanism. Hereinafter, the wavelength continuity that is variable in this way is referred to as phase continuity. For example, as an example of a conventional wavelength tunable semiconductor laser, there is one as disclosed in JP-A-7-74421.

FIG. 6 shows an example of the structure of a conventional phase-tunable wavelength tunable semiconductor laser. Note that the wavelength tunable semiconductor laser shown in FIG. 6 is similar to that shown in FIG.
Since it is the same as that shown in FIG. 3, detailed description of its operation will be omitted, and only the configuration will be briefly described below.

In FIG. 6, reference numeral 12 is an integrated semiconductor laser having an antireflection film 2 provided on one end surface thereof.
It is configured to be driven by the drive circuit 10 and also capable of adjusting the resonator length by the refractive index variable circuit 13. Between the other end surface C of this integrated semiconductor laser 12 and the diffraction grating 7 is a resonator through the lens 3a, and the laser light excited in this resonator is the surface C described above.
Is output to the optical fiber 6 via the lens 3b, the optical isolator 4 and the condenser lens 5.

In the integrated semiconductor laser 12 shown in FIG. 6, the light emitted from the non-reflection film 2 side is converted into parallel light by the lens 3a and enters the diffraction grating 7. The diffraction grating 7 is fixed to one end B of a rod 8, and one end B and the other end A of the rod 8 can move on a Y-axis and an X-axis that are orthogonal to the optical axis, respectively. 9a, 9
It can be moved by b. The parallel movement mechanism 9 a on the Y axis includes a resonator length adjustment mechanism 11.

[0009]

Generally, the semiconductor laser (12) has an optical gain over a wide wavelength range, but as shown in FIG. 7, its value changes gently depending on the wavelength. On the other hand, the optical loss of the resonator in the wavelength tunable semiconductor laser is almost constant with respect to the wavelength as shown in FIG. On the other hand, since the oscillation gain is obtained by subtracting the optical gain from the optical loss, the threshold current, which is almost proportional to the oscillation gain, has a characteristic opposite to that of FIG. 7, as shown in FIG.

When the threshold current further increases, the refractive index of the semiconductor laser decreases, so that the amount of change in the refractive index due to the oscillation gain has the characteristic shown in FIG. At this time
The resonator length changes with wavelength in proportion to 0, which causes a non-linear error. However, the above explanation is qualitative,
For example, when the threshold current is measured and the nonlinear error is estimated, it cannot be accurately performed.

Of course, it is possible to set an adjusting mechanism other than the resonator length adjusting mechanism 11. Further, it becomes possible to use the refractive index variable circuit 13 so that the refractive index value becomes constant irrespective of the wavelength, and by doing so, the adjustment resolution becomes higher than that of the resonator length adjusting mechanism 11.

However, in any case, the non-linear error cannot be canceled by these adjustments. So conventionally,
For example, there may be a method of performing optical correction for each wavelength by the refractive index variable circuit 13 so as to further widen the phase continuous wavelength tunable width while performing mechanical adjustment, storing the value, and further performing correction for each wavelength tunable. Has been. However, in this method, there is no guarantee that the correction value is the optimum value because the adjustment takes time and the mode does not skip.

As described above, conventionally, in order to widen the phase continuous wavelength tunable range, the nonlinear error is estimated and the correction is performed for each wavelength. Therefore, it takes a lot of time for adjustment. Was inaccurate.

The present invention has been made under the background as described above, and provides a wavelength tunable semiconductor laser capable of expanding the phase continuous wavelength tunable width without requiring correction for each wavelength and only by simple mechanism adjustment. The purpose is to do.

[0015]

In order to solve the above-mentioned problems, in the invention according to claim 1, one end face is an output side resonator mirror, and both ends of the one end face and the other end face are provided. A semiconductor laser oscillating means for emitting laser light and an optical axis of the laser light on the other end face side are arranged, and a resonator is formed by the semiconductor laser oscillating means, and the laser light is incident at a predetermined angle. A diffraction grating reflecting on the optical axis,
A parallel moving unit that changes the angle of the diffraction grating and the resonator length on the optical axis, and the optical axis between the semiconductor laser and the diffraction grating are inserted so that the attenuation amount is changeable. Optical attenuator, branching means for branching the laser light emitted from the output side resonator mirror of the semiconductor laser oscillation means into two, and one of the laser light branched by the branching means is incident, And a control unit for controlling the amount of attenuation of the optical attenuator based on the intensity of the incident laser light.

Further, in the invention according to claim 2,
One end face is an output side resonator mirror, and semiconductor laser oscillation means for emitting laser light to both sides of the one end face and the other end face, and the laser light emitted from the other end face of the semiconductor laser oscillation means are incident. From a resonator, an optical attenuator that is inserted on the optical axis between the semiconductor laser and the resonator, and has an adjustable attenuation amount, and from the output-side resonator mirror of the semiconductor laser oscillation means. The emitted laser light is 2
It comprises: a branching unit for branching, and one of the laser beams branched by the branching unit, and a control unit for controlling the attenuation amount of the optical attenuator based on the intensity of the incident laser beam. To do.

In the invention according to claim 3,
3. The wavelength tunable semiconductor laser according to claim 2, wherein the resonator faces a half mirror on which the laser light is incident, on the axis orthogonal to the optical axis of the incident laser light with the half mirror interposed therebetween. A total reflection mirror, and a diffraction grating which makes the laser light incident at a predetermined angle and reflects on the orthogonal axis,
One of the laser beams incident on the half mirror from the semiconductor laser oscillating means is composed of a parallel moving means for moving the total reflection mirror in parallel in the orthogonal axis direction and a rotating means for changing the angle of the diffraction grating. Part is reflected to the total reflection mirror side, and part of the laser light reflected by the total reflection mirror is reflected to the optical axis side by the half mirror and part of the laser light is incident on the diffraction grating. However, a part of the laser light reflected by the diffraction grating is transmitted through the half mirror.

Further, in the invention according to claim 4,
The tunable semiconductor laser according to any one of claims 1 to 3, wherein the control unit controls an attenuation amount of the optical attenuator so that the incident laser light intensity has a constant value. .

[0019]

According to the present invention, a part of the laser light emitted from the one end face of the semiconductor laser oscillating means and branched by the branch portion is incident on the control portion, and the control portion has a constant intensity of the incident laser light. Therefore, the attenuation amount of the resonator or the optical attenuator inserted between the resonator and the semiconductor laser oscillation means is controlled.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

A. First Embodiment Hereinafter, a first embodiment of the present invention will be described.
FIG. 1 is a configuration diagram showing the configuration of a wavelength tunable semiconductor laser according to the first embodiment of the present invention. Note that, in FIG. 1, portions corresponding to the respective portions shown in FIG. 6 are denoted by the same reference numerals, and description thereof will be omitted.

In FIG. 1, reference numeral 1 is a semiconductor laser, and 30 is an optical attenuator inserted in a resonator formed between the semiconductor laser 1 and the diffraction grating 7. In the present embodiment, as the optical attenuator 30, for example, a glass plate on which a metal film is deposited while changing the film thickness is used.
The attenuation amount of 0 is controlled by the control unit 32 described later.

Reference numeral 31 is an optical demultiplexer, which splits the laser light output from the semiconductor laser 1 through the lens 3b and the optical isolator 4 and makes the laser light incident on the lens 5 and the controller 32. Further, 32 is a control unit, which controls the attenuation amount of the optical attenuator so that the incident light intensity has a constant value.
This control unit 32 is a kind of automatic light output control, and controls the threshold current to be constant by controlling the optical loss in the resonator,
Control is performed so that the nonlinear error ΔL2 (θ) (described later) becomes zero.

In the configuration shown in FIG. 1, the normal line N of the diffraction grating 7 and the optical axis form an angle π-θ [rad]. At this time, corner O
AB is the angle θ. Here, assuming that the wavelength corresponding to the maximum reflectance of the diffraction grating 7 is λGR (θ) as a function of the angle θ, and the grating interval of the diffraction grating 7 is d, λGR (θ) is calculated from the Bragg diffraction condition. = 2 · d · sin θ (1) Similarly, the wavelength of the oscillation longitudinal mode proportional to the cavity length is λFP (θ) as a function of the angle θ, and the mode order is m. If the length of the rod 8 (that is, the line segment AB) is L0, then λFP (θ) = 2 {L0 · sin θ−L (θ)} / m (2)

Here, the difference between the actual resonator length and L0 · sin θ is set to L (θ). In this case, if the refractive index is not taken into consideration, L (θ) is constant due to the line segment OC, but since the refractive index actually changes depending on the wavelength, L (θ) is the wavelength θ.
Is a function of Optically, the cavity length is the product of the physical length and the refractive index, so it is necessary to consider the refractive index component for optical components, for example, the physical length is 300 μm and the refractive index is 3.3.
The cavity length of the semiconductor laser is 300 μm × 3.3 = 9
It is 90 μm. The same applies to the following, and the resonator length is expressed in terms of refractive index unless otherwise specified.

Here, from equation (2), the mode interval is λFP /
m. This is also a measure of mode jump, and the difference between equations (1) and (2), that is, λGR-λFP is the mode interval λFP / m.
It can be considered that mode jump occurs when the value exceeds.

Therefore, as an initial condition, it is assumed that λGR-λFP = 0 when θ = θ0. That is, λGR (θ0) = λFP (θ0) (3). Next, consider a case where there is a change in wavelength Δλ and a change in angle Δθ. Here, the relationship between Δλ and Δθ is calculated as Δλ = 2 · d · cos θ0 · Δθ (4).

In the equation (4), if Δλ is about 100 nm, the first-order approximation of Δθ is established. Therefore, from the equations (1) to (3),

[Equation 1] Becomes Here, L (θ0 + Δθ) = L (θ0) + ΔL
I said.

Further, ΔL is due to the change in the refractive index due to the change in wavelength. However, the refractive index of optical components such as lenses is proportional to the wavelength in the wavelength range of about 100 nm. On the other hand, since the change in the refractive index due to the change in the oscillation gain of the semiconductor laser due to the change in wavelength is a non-linear value that is not proportional to the wavelength, ΔL is further expressed below. ΔL = L1 · Δθ + ΔL2 (θ) (6) Substituting equation (6) into equation (5),

[Equation 2] Should be adjusted so that the value is within the mode interval λFP / m.

Further, in the equation (7), L (θ0) is adjusted by the resonator length adjusting mechanism 11 so as to be canceled by the term of L1, and −2 · L (θ0) / (m ・
It is sufficient to bring the part of (tan θ0) + 2 · L1 / m close to zero.

That is, the control unit 32 causes the optical attenuator 30 to change the optical loss of the resonator as shown in FIG. 2 in accordance with the optical gain. As a result, the threshold current becomes constant as shown in FIG. 3, and therefore, as shown in FIG. 4, the amount of change in the refractive index due to the oscillation gain can be set to 0, that is, ΔL2 (θ) = 0.

As described above, if the optical loss of the resonator is controlled so that the optical output becomes constant while keeping the driving current of the semiconductor laser constant, the threshold current also becomes constant, and ΔL2 (θ) = 0 can be realized. By performing the mechanism adjustment in this state, the phase continuity is relatively easily improved.

B. Second Embodiment Next, a second embodiment of the present invention will be described.
FIG. 5 is a configuration diagram showing a configuration of a wavelength tunable semiconductor laser according to the second embodiment of the present invention. In FIG. 5, parts corresponding to those shown in FIG. 1 or 6 are designated by the same reference numerals, and their description will be omitted.

In FIG. 5, 21 is a half mirror, 22 is a total reflection mirror, and 23 is a rotating mechanism for rotatably supporting the diffraction grating 7 around the point B. Furthermore, 24 is a parallel movement mechanism, and in this embodiment, the point C moves. In the present embodiment, an optical resonance reflecting mirror is formed between the diffraction grating 7 and the total reflection mirror 22, and the half mirror 21 is inserted between them.

In the configuration shown in FIG. 5, the half mirror 2
When the position of 1 is S and the output side end face of the semiconductor laser 1 is B ', the physical resonator length is the line segment CS + SB'. The line segment CB is the physical resonator length of the optical resonance mirror. According to the present embodiment, with the above-mentioned configuration, it is possible to easily tune the broadband phase continuous wavelength in the external resonator type semiconductor laser using the optical resonance reflecting mirror capable of narrowing the line width even with a short resonator length.

In each of the above-mentioned embodiments, the optical attenuator 30 is exemplified by a glass plate on which a metal film is vapor-deposited while changing the film thickness. It may be.

[0036]

As described above, according to the present invention,
A part of the laser light emitted from one end face of the semiconductor laser oscillating means and branched by the branching portion is made incident on the control portion, and the control portion makes a resonator or a resonance so that the intensity of the incident laser light becomes a constant value. Since it controls the amount of attenuation of the optical attenuator inserted between the laser and the semiconductor laser oscillation means, it takes time and eliminates non-linear errors without inaccurate correction for each wavelength. Thus, there is an effect that a wavelength tunable semiconductor laser capable of expanding the phase-continuous wavelength tunable width can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a schematic configuration of a wavelength tunable semiconductor laser according to an embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between the wavelength of laser light and the optical loss of a resonator in the same embodiment.

FIG. 3 is a diagram showing a relationship between a wavelength of laser light and a threshold current of a resonator in the same embodiment.

FIG. 4 is a diagram showing the relationship between the wavelength of laser light and the amount of change in the refractive index of the resonator in the same embodiment.

FIG. 5 is a configuration diagram showing a schematic configuration of a wavelength tunable semiconductor laser according to another embodiment of the present invention.

FIG. 6 is a configuration diagram showing a schematic configuration of a wavelength tunable semiconductor laser according to a conventional technique.

FIG. 7 is a diagram showing the relationship between the wavelength of laser light and the optical gain of a semiconductor laser in the prior art.

FIG. 8 is a diagram showing the relationship between the wavelength of laser light and the optical loss of a resonator in the prior art.

FIG. 9 is a diagram showing a relationship between a wavelength of laser light and a threshold current of a resonator in a conventional technique.

FIG. 10 is a diagram showing the relationship between the wavelength of laser light and the amount of change in the refractive index of the resonator in the prior art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 7 Diffraction grating 8 Rod 9a, 9b Parallel moving mechanism 11 Resonator length adjusting mechanism 24 Parallel moving mechanism 30 Optical attenuator 31 Optical demultiplexer 32 Control part

Claims (4)

[Claims] 1. A semiconductor laser oscillating means (1), one end surface of which is an output side resonator mirror, which emits laser light to both sides of the one end surface and the other end surface, and an optical axis of the laser light on the other end surface side. A diffraction grating (7) which is arranged on the optical axis, forms a resonator with the semiconductor laser oscillating means, allows the laser light to enter at a predetermined angle, and reflects the laser light on the optical axis; A parallel moving means (9a, 9b) for changing the angle of the grating and the resonator length, and a light which is inserted on the optical axis between the semiconductor laser and the diffraction grating and whose attenuation amount is changeable. Attenuator (3
0) and a branching means (31) for branching the laser light emitted from the output side resonator mirror of the semiconductor laser oscillation means into two.
And one of the laser beams branched by the branching unit, and a control unit (32) for controlling the attenuation amount of the optical attenuator based on the intensity of the incident laser beam. Tunable semiconductor laser. 2. A semiconductor laser oscillating means (1), one end surface of which is an output-side resonator mirror, which emits laser light to both sides of the one end surface and the other end surface, and the other end surface of the semiconductor laser oscillating means. And an optical attenuator (3) which is inserted on the optical axis between the semiconductor laser and the resonator and in which the attenuation amount is changeable.
0) and a branching means (31) for branching the laser light emitted from the output side resonator mirror of the semiconductor laser oscillation means into two.
And one of the laser beams branched by the branching unit, and a control unit (32) for controlling the attenuation amount of the optical attenuator based on the intensity of the incident laser beam. Tunable semiconductor laser. 3. The half mirror (21) into which the laser light is incident, and the total reflection mirror (21) facing the optical axis of the incident laser light with the half mirror interposed therebetween. 22), and a diffraction grating (7) which makes the laser light incident at a predetermined angle and reflects it on the orthogonal axis, and a parallel moving means (24) for moving the total reflection mirror in parallel in the orthogonal axis direction, Rotating means (23) for changing the angle of the diffraction grating, and a part of the laser light incident on the half mirror from the semiconductor laser oscillating means is reflected toward the total reflection mirror side, and is reflected by the total reflection mirror. A part of the reflected laser light is reflected by the half mirror toward the optical axis and a part of the reflected laser light is incident on the diffraction grating, and the laser light reflected by the diffraction grating is partially. Claim 2, characterized by transmitting the half mirror
The tunable semiconductor laser according to item 1. 4. The wavelength according to claim 1, wherein the controller controls an attenuation amount of the optical attenuator so that the incident laser light intensity has a constant value. Tunable semiconductor laser.