JPH09260792A - External resonator-type wavelength-variable ld light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize the oscillation frequency varying, owing to the temp. variation over the entire wavelength variable range, without optical output variation or mechanical backlash. SOLUTION: On an external resonator type LD (wavelengthvariable semiconductor laser) light source, a diffraction grating 21 is disposed as an external reflection mirror of an LD 233 having an RA coat 232 at one end face. A glass board 22 is inserted in a resonator composed of the grating 21 and LD 232. Temp. detector elements 24 and 221 detect the temps. of an optical base 2 and board 22. The resonator length change, due to the temp. change of the base 2, is corrected by controlling the temp. of the board 22, using a glass board temp. control circuit 3 and Peltier element 222, thus stabilizing the oscillation wavelength of the LD light source.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external resonator type wavelength tunable semiconductor laser (LD) light source, and more particularly to a wavelength of an external resonator type wavelength tunable LD light source used in the field of optical coherent communication and measurement technology. It is about stabilization technology.

[0002]

2. Description of the Related Art For example, in optical coherent communication / measurement technology, it is necessary to use an LD light source of single mode oscillation capable of wavelength tuning in a wide range, having a narrow spectral line width and good wavelength stability. Note that such a wavelength tunable has a variable width of 100 nm, a resolution of 1 pm, and a wavelength stability of 0.1.
The market is demanding that the pm can be realized and the size is small.

An external resonator type LD light source using a diffraction grating or the like is generally used as the single mode oscillation LD light source capable of wavelength tuning in a wide range as described above. The oscillation wavelength operation of the external resonator type LD light source using such a diffraction grating will be briefly described below.

First, the oscillation wavelength of the LD light source is given by the following equations (1) and (2). in this case,
The LD light source oscillates near the Bragg wavelength λ _b of the diffraction grating at a wavelength with a small mirror loss and with a phase condition due to the optical resonator.

Λ _M = 2 × n × L / M (1) λ _b = 2 × d × sin θ / m (2) where λ _M : Resonant wavelength in the external resonator M: In the external resonator longitudinal mode number (an integer), L: the length of the external resonator, n: the refractive index of the external cavity, lambda _b: Bragg wavelength, d: groove spacing of the grating (grating constant), theta: angle of incidence to the diffraction grating ( Litrow arrangement), m: Order of reflected light of the diffraction grating (usually m = 1).

As described above, in the LD light source using the diffraction grating, the oscillation wavelength of the light source can be tuned by changing the angle of the diffraction grating and changing the incident angle θ to the diffraction grating. However, when the incident angle θ is constant, the refractive index n and the resonator length L in the equation (1) change depending on the temperature, so that the oscillation wavelength λ _M changes due to the temperature change.
Here, the fluctuation of the oscillation wavelength due to the temperature fluctuation of the LD is
Since the fluctuation of the physical length of is smaller than the fluctuation of the refractive index of the LD, the following expression (3) is obtained.

[0007] _{Δλ M = (λ M / L} ) × Note _{(Δn LD × L LD) ...} (3), Δλ M: the oscillation wavelength variation, L _LD: physical length of the LD, [Delta] n _LD: the refractive index fluctuation of LD is there.

Here, λ _M = 1550 nm, L = 30 mm, L _LD
= 300 μm and Δn _LD = 2 × 10 ^-4 (° C ^-1 ),
According to the equation (3), Δλ _M changes by about 3 pm per 1 ° C. temperature change of the LD. Considering what is required in the market, the temperature fluctuation of the LD needs to be within about 0.033 ° C.

By the way, in an external resonator type wavelength tunable LD light source using a diffraction grating, as a structure for stabilizing the oscillation wavelength variation due to temperature variation in the prior art, for example, shown in FIGS. 3, 4 and 5. There is a configuration.

FIG. 3 shows a structure in which the wavelength is stabilized by controlling the temperature of the entire light source resonator section with high accuracy. In this case, the LD 233 having the AR coat 232 on one end surface
And the diffraction grating 2 as an external reflecting mirror on the end face side of the AR coat
1 and 1 form an optical resonator. The LD unit 23 including the LD 233, the lenses 231, 234, and the optical isolator 235 is equipped with a temperature detection element 236 and a Peltier element 227. The LD section 23 is controlled by the temperature control circuit 5, and the LD section 23 and the diffraction grating 21 are mounted on the optical system base 2. Also,
The LD drive circuit 1 controls the LD drive current to be constant.
Further, the optical system base 2 is equipped with a temperature detecting element 221 and a Peltier element 222, which is controlled by the temperature control circuit 5 with high accuracy to stabilize the wavelength.

However, in order to control the temperature of the entire optical system on the optical system base 2 by the configuration of FIG. 3, since the optical system base 2 is large, the entire optical system on the optical system base 2 is controlled. It is very difficult to control the temperature uniformly. Therefore, the configuration of FIG. 3 is not practical.

On the other hand, FIG. 4 shows a structure in which the temperature of the LD is controlled and the refractive index of the LD is changed to change the external resonator length and stabilize the wavelength. In this case, each member mounted on the optical system base 2 and the LD drive circuit 1 are the same as those in FIG. Note that FIG. 6 shows the main part of FIG. In FIG. 6, n ₀ , n _LD , and n indicate the refractive index.

In the structure of FIG. 4, as a wavelength stabilizing method, a part of the output light is taken out and input to the wavelength measuring unit 6 provided outside the light source to measure the oscillation wavelength.
A configuration is adopted in which the measured wavelength is input to the CPU 41. Then, the CPU 41 inputs a signal proportional to the fluctuation of the oscillation wavelength to the temperature control circuit 5, and the temperature control circuit 5 outputs CP.
The temperature of the LD 233 is changed by the signal from U41. As a result, it is possible to stabilize the oscillation wavelength by compensating for the change in the resonator length that has changed due to the thermal expansion of the optical system base 2. If the right side of the above equation (1) is written in detail, it becomes like the following equation (4) from FIG.

Λ _M = 2 × {(n _LD × L _LD ) + (n ₀ × (L−L _LD ))} / M (4) where n ₀ : refractive index of air (= 1), n _LD : The refractive index of LD.

However, in the case of the structure of FIG. 4, the refractive index n _LD of the LD of the formula (4) is changed by changing the temperature of the _LD.
To stabilize the wavelength, but when the temperature of the LD is changed, the threshold current of the LD changes in FIG. In the laser oscillation region, LD
Since the light output P with respect to the drive current I is expressed by the following equation (5), when the drive current is constant, the light output fluctuates due to the fluctuation of the threshold current. Therefore, in the configuration in which the oscillation wavelength is stabilized by changing the temperature of the LD, the threshold current fluctuates due to the temperature change, and as a result, the optical output fluctuates.

P = η _d × (hν / q) × (I−I _th ) ... (5) where P is the optical output in the laser oscillation region, η _{d is the} external differential quantum efficiency, and hν is the energy of light. , Q: charge of injected carriers, I: drive current of LD, I _th : threshold current of LD.

The configuration of FIG. 5 is a configuration in which the oscillation wavelength is stabilized by mechanically changing the length of the external resonator. In this configuration, as in FIGS. 3 and 4, the LD 233 and the diffraction grating 21 form an optical resonator. LD section 23
Is equipped with a temperature detection element 236 and a Peltier element 237, which are controlled by the temperature control circuit 5.
The temperature of the LD 233 is stabilized. Then, the LD drive circuit 1 controls the LD drive current to be constant. Further, the diffraction grating 21 is attached to the rotary table 213, the rotary table 213 is attached to the translation table 214, and the LD unit 23 and the translation table 214 are mounted on the optical system base 2. . The configuration for stabilizing the wavelength is the same as the configuration in FIG. 4 until a part of the output light is extracted and the oscillation wavelength variation is calculated by the CPU 41. The difference is that the resonator length control circuit 8 is a CPU
The point is that the linear movement table 214 is moved in the optical axis direction by the signal from 41 to correct the variation of the resonator length changed by the thermal expansion of the optical system base 2 to stabilize the oscillation wavelength.

[0018]

By the way, since the temperature of the LD is stabilized in the case of the structure of FIG. 5 described above,
There is no problem of fluctuation of optical output due to fluctuation of threshold current of LD. However, since the linear motion table is mechanically moved, mechanical backlash inevitably occurs, which causes a shift in the external cavity length, resulting in poor wavelength stability. There is a problem.

Further, in the configuration of FIGS. 4 and 5, a Fabry-Perot etalon or an optical frequency counter is used in the wavelength measuring section. The Fabry-Perot etalon requires high-precision temperature control and resonance of the Fabry-Perot etalon. There is a problem that the wavelengths other than the peak wavelength of the Fabry-Perot etalon cannot be stabilized because of the resonance peak wavelength interval determined by the device length. Further, in the case of the optical frequency counter, there is a problem that it is very expensive and becomes large in size.

Therefore, an object of the present invention is to provide an external resonator type LD light source capable of stabilizing oscillation wavelength variation over the entire wavelength tunable range without optical output variation or mechanical backlash.

[0021]

An external resonator type wavelength tunable LD light source according to the present invention includes a semiconductor laser, a diffraction grating as an external reflecting mirror, and a resonator composed of the diffraction grating and the semiconductor laser. The inserted substrate, the first temperature detecting element provided on the substrate for detecting the substrate temperature, the temperature control element for changing the temperature of the substrate, the semiconductor laser, the diffraction grating, the substrate, and the first substrate. No. 1 temperature detecting element and an optical system base on which the temperature control element is mounted, and a second temperature detecting element for detecting the temperature of the optical system base, and the temperature fluctuation of the optical system base. The oscillation wavelength is stabilized by controlling the temperature of the substrate based on the above.

That is, in the present invention, the temperature fluctuation of the optical system base on which the optical system of the external resonator type LD light source is assembled is detected, and the diffraction grating and the LD are detected based on the detected result.
The temperature of the substrate, which is an optical element with a temperature-dependent refractive index, inserted in the cavity made up of is controlled and the refractive index is changed. The amount of change in the external resonator length is corrected and controlled to stabilize the wavelength of the light source.

[0023]

Embodiments of the present invention will be described below. FIG. 1 shows an external resonator type wavelength tunable LD light source according to an embodiment of the present invention.

In FIG. 1, an AR coat 232 is formed on one end surface.
Light emitted from the AR coat end surface side of the Fabry-Perot type LD 233 subjected to the above is converted into parallel light by the lens 231. As the lens 231, an aspherical lens or a combination lens having a small wavefront aberration is preferably used. Then, a temperature detecting element 236 such as a thermistor or a thermocouple and a Peltier element 237 are attached to the LD section 23 composed of the LD 233, the lenses 231, 234 and the optical isolator 235, and an LD temperature control circuit using these elements. The LD unit 23 is controlled by 5. In the LD temperature control circuit 5, the oscillation wavelength fluctuation is 0.1 p
The temperature of the LD is stabilized within 0.033 ° C. to keep the temperature below m.

Further, the diffraction grating 21 is provided as an external reflecting mirror on the AR-coated side of the LD 233, and an optical resonator is formed by this and the LD end face which is not AR-coated. In addition, the diffraction grating 2 in this optical resonator
A glass substrate 22 provided with a temperature detection element 221 and a Peltier element 222 is inserted between the 1 and the LD section 23.

The LD section 23, the diffraction grating 21 and the glass substrate 22 are mounted on the optical system base 2 provided with the temperature detecting element 24. Further, the glass substrate temperature control circuit 3 controls the Peltier element 222 of the glass substrate 221 by the temperature detection signal detected by the temperature detection element 221 of the glass substrate 22 to stabilize the temperature of the glass substrate 22. . The coefficient of thermal expansion of the resonator length corresponds to the coefficient of thermal expansion of the optical system base table 2. Specifically, when the invar material is used as the optical system base table, the value of the coefficient of thermal expansion is room temperature. It is 0.13 × 10 ⁻⁶ at ° C.

The temperature detecting element 24 of the optical system base 2 is
The temperature of the optical system base 2 is detected. This detection signal is input to the control unit 4. The control unit 4 stores the temperature of the optical system base 2 when the wavelength stabilization is started, and
A signal proportional to the subsequent temperature fluctuation is output to the glass substrate temperature control circuit 3. In addition, by the signal from the control unit 4,
The glass substrate temperature control circuit 3 changes the temperature of the glass substrate and stabilizes the glass substrate again by this temperature, thereby stabilizing the oscillation wavelength variation due to the temperature variation of the optical system base 2.

Here, the oscillation wavelength fluctuation amount Δλ _M when the temperature of the LD 233 is constant and the temperatures of the optical system base 2 and the glass substrate 22 fluctuate is given by the following equation (4) and FIG. Like

Δλ _M = (Δn _G × L _G + n ₀ × ΔL ₀ ) × λ _M / (n _LD × L _LD + n _G × L _G + n ₀ × L ₀ ) (6) where L _G : glass substrate Thickness (= 2 × 10 ⁻³ mm), L ₀ : L ₀₁ + L _{02 in} FIG. 2, n _G : Refractive index of glass substrate (= 1.4563), Δn _G : Change in refractive index of glass substrate, ΔL ₀ : This is the amount of change in L ₀ .

Then, in order to eliminate the wavelength variation, Δλ _M = 0 may be set in the above equation (6). Also, ΔL
₀ indicates that the temperature of the LD 233 is constant, so the optical system base 2
Is equal to the amount of change in the resonator length due to the temperature fluctuation of. Therefore,
If L = 30 mm, then ΔL ₀ =
3.9 nm / ° C, and Δn _G = -1.95 from the above formula (6)
It becomes × 10 ^-6 ° C ^-1 .

Since the change in the refractive index of the glass substrate 22 due to the change in the temperature of the glass substrate 22 is −3 × 10 ⁻⁶ ° C. ⁻¹ , the temperature of the glass substrate 22 is changed with respect to the temperature fluctuation of 1 ° C. of the optical system base 2. By changing about 0.6 ° C, Δλ _M =
It becomes 0.

Further, in order to keep the oscillation wavelength variation within 0.1 pm, n _LD = 3.54 and L _LD = 300 μm,
Δn _G ≦ −0.93 × 10 ⁻⁶ . Therefore, the glass substrate 2
The temperature fluctuation of 2 may be suppressed to within 0.3 ° C. And
As a result, the temperature control of LD233 was 0.033 ℃,
Equivalent wavelengths can be stabilized by temperature control of about 1/10.

[0033]

As described above, according to the present invention, the change in the resonator length due to the thermal expansion due to the temperature fluctuation of the optical system base table in which the resonator (external resonator) is constituted is constituted by the diffraction grating and the LD. Since the oscillation wavelength of the LD light source is stabilized by changing and correcting the refractive index of the substrate inserted in the resonator, there is no optical output fluctuation and there is no problem of mechanical backlash. In addition, it is not necessary to use a reference optical element such as a Fabry-Perot etalon, and the wavelength can be stabilized at any wavelength.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an embodiment of an external resonator type wavelength tunable LD light source according to the present invention.

FIG. 2 is an explanatory diagram of a main part of FIG. 1;

FIG. 3 is an explanatory diagram of an LD light source in which the wavelength is stabilized by controlling the temperature of the LD light source unit in the related art.

FIG. 4 is an explanatory view of another LD light source in which the wavelength is stabilized by controlling the temperature of the LD light source unit in the prior art.

FIG. 5 is an explanatory view of an LD light source in which a wavelength is stabilized by mechanically changing an external resonator length in a conventional technique.

FIG. 6 is an explanatory diagram of a main part of FIG.

FIG. 7 is a graph showing a current-light output characteristic curve of an LD.

[Explanation of symbols]

 1 LD drive circuit 2 Optical system base 3 Glass substrate temperature control circuit 4 Control part 21 Diffraction grating 22 Glass substrate 23 LD part 24 Temperature detection element

Claims (4)

[Claims] 1. A semiconductor laser (233), a diffraction grating (21) as an external reflecting mirror, and a substrate (22) inserted in a resonator composed of the diffraction grating (21) and the semiconductor laser (233). ), A first temperature detection element (221) provided on the substrate (22) for detecting the substrate temperature, a temperature control element (222) for changing the temperature of the substrate (22), a semiconductor laser (233), Diffraction grating (21), substrate (22), first
The optical system base (2) on which the temperature detecting element (221) and the temperature control element (222) are mounted, and the second temperature detecting element (24) for detecting the temperature of the optical system base (2). An external resonator type wavelength tunable LD light source, characterized in that the oscillation wavelength is stabilized by controlling the temperature of the substrate (22) based on the temperature fluctuation of the optical system base (2). 2. The external resonator type wavelength tunable LD light source according to claim 1, wherein the substrate (22) is a glass substrate (22). 3. A temperature control element (222) is a Peltier element (22).
2) The external resonator type wavelength tunable LD light source according to claim 1 or 2. 4. The semiconductor laser (233) is provided with an AR coat (232) on the side of the diffraction grating (21).
The external resonator type wavelength tunable LD light source described.