JPH0542149B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an external resonator type semiconductor in which rear emitted light from a semiconductor laser device is reflected by an external reflecting member (mirror) and returned to the inside of the semiconductor laser device. This invention relates to a laser device.

<Prior Art> The oscillation axis mode of a conventional semiconductor laser is selected depending on the gain distribution of the laser medium and the transmission characteristics of the laser resonator. FIG. 6 is a diagram showing the oscillation axis mode selectivity of a conventional semiconductor laser, and FIG.
Figure b shows the gain distribution of the laser medium with respect to wavelength (horizontal axis), Figure b shows the spectrum of each axial mode with respect to wavelength, and Figure c schematically shows the spectrum of the super radiant state where the above a and b are superimposed. It is shown in Among the various axial modes of the laser, the one with a wavelength close to the peak (maximum value) of the gain distribution obtains the maximum gain and becomes the oscillation axial mode. However, as the ambient temperature changes, the band gap of the semiconductor changes, causing the gain distribution to change. The peak wavelength of changes to the longer wavelength side at a rate of 2 to 3 Å/deg. Furthermore, since the refractive index of the medium changes and the laser element itself also thermally expands, the effective optical length of the laser resonator changes, and as a result, each axial mode maintains a spacing of approximately 3 Å, while the laser element itself thermally expands. It changes to the longer wavelength side at a certain rate.
Therefore, when the temperature is raised beyond a certain state, the oscillation wavelength changes continuously for a while because the amount of change in the gain distribution is larger than the amount of change in the axial mode, but eventually mode hopping occurs, and after that, the oscillation wavelength changes continuously for a while. As shown in the figure, continuous changes and mode hopping are repeated, and changes occur on a staircase. In addition, the wavelength changes depending on the current value used to drive the semiconductor laser, which has hindered its application as a light source for wavelength multiplexed optical communications and high-resolution spectroscopy, which are expected in the future.

Therefore, SEC laser (Short External Cavity
A laser diode was invented, which reflects the backward emitted light of a semiconductor laser using an external mirror.
The oscillation axis mode is fed back to the semiconductor laser main body.
The selection is based on two factors. Figure 7 schematically shows this situation in correspondence with Figure 6.
Figure a shows the gain distribution of the laser medium with respect to wavelength, and figure b shows the spectrum of each axial mode with respect to wavelength.
Figure c shows the resonance characteristics of the external resonator with respect to wavelength.
Figure d shows the spectrum of the super radiant state in which a, b, and c are superimposed. The spectral envelope in the superradiant state is the 6th
Unlike the case shown in the figure, it has ripples as shown in FIG. 7d. In this case, since the temperature characteristic of the peak of the envelope can be controlled by changing the external cavity length, that is, the gap length between the semiconductor laser and the external mirror, mode hops can be suppressed.

Typical examples of the characteristics of the oscillation wavelength with respect to temperature of this SEC laser are shown in Figures 9a, b, and c. In each case, the same axial mode is maintained in the temperature range Δt, and in the temperature range ΔT, Figure 7 At the same peak of the spectrum envelope shown in d, the axial mode sequentially gains maximum gain and becomes the oscillation axis mode, and when ΔT is exceeded, the oscillation axis mode shifts to the peak of the next peak of the envelope, creating a large mode hop. .

Furthermore, FIG. 9a shows that for the temperature coefficient d/dT of the peak wavelength of the envelope shown in FIG. 7d and the temperature coefficient γ of the axial mode shown in FIG.
When γ, the oscillation axis mode sequentially shifts to the adjacent axis mode on the shorter wavelength side, and a small mode hop occurs within the range of ΔT.
This shows a state in which ΔT=Δt when λ/dT=γ, and only large mode hops occur. Further, FIG. 9c shows a state in which, when d/dT>γ, the oscillation axis mode sequentially shifts to the adjacent axis mode on the longer wavelength side, producing a small mode hop.

<Problems to be solved by the invention> Conventional SEC lasers are made of GaAs-GaAlAs, for example.
In the case of a system, a double heterojunction semiconductor laser is made of a GaAs substrate with a VSIS structure stacked with AlGaAs as the active layer, and an external reflective member of a GaAs chip with a dielectric coating on the cleaved end face to provide high reflectance. The external resonator is fixed to a mounting table at a distance of a predetermined external resonator length (the distance between the emission end face of the semiconductor laser and the mirror reflection surface of the external reflection member). At this time, the length of the GaAs chip serving as the reflective member in the direction of the laser cavity was set to be equal to or longer than the cavity length of the semiconductor laser. This is primarily due to manufacturing constraints, namely the ease of handling GaAs chips. When the length of the GaAs chip is 300 μm, if you want to match d/dT with the temperature coefficient of the axial mode (0.7 Å/deg) and obtain the characteristics shown in Figure 9b, the external cavity length should be set to 65 μm. At this time, ΔT = 26℃ as shown in the same figure.
It is possible to prevent mode hops from occurring within the range of . However, it is difficult to arbitrarily control the temperature at which mode hops occur (the temperature at both ends of the range of ΔT shown in Figure 9b), and even when using a semiconductor laser device with such characteristics, there is a temperature at which no mode hops occur at all. If the range is limited to a specific temperature range for use, the yield will decrease and it will take time to select high-quality elements.
In order to solve this problem, it is effective to make the temperature range ΔT in which mode hops do not occur as large as possible.

An object of the present invention is to provide an improved technique for the external resonator structure to further increase the above-mentioned ΔT.

<Summary of the Invention> FIG. 1 is an explanatory diagram for explaining the basic principle of the present invention, in which 1 is a semiconductor laser element, and 2 is a semiconductor laser that emits laser light emitted from one emission end face 1a of the semiconductor laser element 1. 1 is a reflecting member having a reflecting surface 2a for returning the semiconductor laser element 1 and the reflecting member 2, and 3 is a mounting member on which the semiconductor laser element 1 and the reflecting member 2 are placed and fixed. l is the distance between the emission end face 1a and the reflection surface 2a (external cavity length), L is the length of 1/2 of the total cavity length of the semiconductor laser element 1, and M is the distance from the center of the reflection member 2 to the reflection surface 2a. distance, D is the semiconductor laser element 1
is the distance between the center of the reflective member 2 and the reflective member 2
The length 2M is the cavity length 2L of the semiconductor laser element 1.
It is configured as follows:

<Operation> As already mentioned, the behavior of the oscillation mode of an external cavity semiconductor laser is determined by three factors: the gain distribution of the laser, the axial mode of the laser, and the wavelength selectivity of the external cavity. Regarding the temperature change of the oscillation mode, by setting appropriate conditions after considering the temperature dependence of these three factors, it is possible to obtain good characteristics in which no mode hops occur in the temperature range of ΔT, as shown in Figure 9b. An element having the following properties is obtained. In order to further widen ΔT, it is effective to set the external resonator length l smaller and widen the resonance interval in the resonance characteristics of the external resonator shown in Figure 7c, but without changing other conditions. If l is simply made smaller, the temperature characteristic dλe/dT of the peak wavelength λe of the resonance characteristic of the external resonator will increase, and the oscillation characteristic of the laser will change to ΔT as shown in Figure 9c.
A mode hop is generated toward the longer wavelength mode within the range of .

The present invention reduces the thermal expansion of the attached external resonator by reducing the thermal expansion area of the reflective member, and also reduces the temperature change of the external mode (resonance peak wavelength of the external resonator) based on this. This is an attempt to expand the stable temperature range of the oscillation wavelength (temperature range in which no mode hops occur). This point will be explained below.

The temperature dependence of the external mode λe is mainly affected by the thermal expansion of the external resonator. Generally, the temperature change rate dλe/dT of the external mode is expressed as dλe/dT = λo/l・dl/dT (1) (where l is the length of the external resonator with λo as the resonant wavelength), and the external mode The smaller the coefficient of thermal expansion of the vessel, the lower the rate of temperature change in the external mode. In the schematic configuration diagram of the laser device shown in FIG. 1, the parts that thermally deform with temperature changes are the semiconductor laser element 1, the reflection member 2, and the mounting member 3, and the external cavity length l is the composite of these thermal deformations. is thought to be caused by temperature changes. The semiconductor laser element 1 and the reflection member 2 are fixed to the mounting member 3 with a fusion material, but due to temperature changes, their centers (X and Y)
It is thought that thermal deformation occurs mainly in Therefore, when considering the temperature change of the external resonator length l, it is sufficient to pay attention to the region having the length D between the two thermal expansion centers (X and Y) as the region where the mounting member is thermally deformed. At this time, the external resonator length l is l=D-(L+M)...(2). If the linear expansion coefficients of the semiconductor laser 1, the reflective member 2, and the mounting member 3 are α ₁ , α ₂ , and α _{3 ,} respectively, then l
The temperature change rate is dl/dT=α ₃ D−(α ₁ L+α ₂ M) = α ₃ L+(α ₃ −α ₁ )L+(α ₃ −α ₂ )M ……(3), so (1 ) The temperature change rate of the external mode is expressed as dλe/dT=λo{α ₃ +L/l (α ₃ −α ₁ )+M/l(α ₃ −α ₂ )} (4).

If the semiconductor laser 1 and the reflective member 2 are made of the same material (α ₁ = α ₂ = α), then equation (4) becomes dλe/dT=λo {α ₃ +L+M/l(α ₃ −α)} …
…(4)′ becomes. Equation (4) shows that the smaller L or M is, the smaller dλe/dT becomes. That is, by reducing the resonator length of the semiconductor laser element fixed on the mounting member or the length (in the resonator length direction) of the reflecting member fixed thereon, the temperature change rate of the external mode can be reduced. The length of the reflecting member referred to herein is the length in the resonator length direction of a region of the reflecting member having a reflecting surface that contributes to moving the reflecting surface due to thermal expansion. As mentioned above, shortening the cavity length of the semiconductor laser device is also effective in reducing dλe/dT, but this is not preferable because it causes an increase in the injection current density and multimode oscillation.

Figure 4 shows the calculation results of equation (4). From the same figure, even if the length of the reflective member is the same, dλe/dT decreases by reducing the length of the reflecting member by 2M, and by decreasing 2M, the same dλe/dT can be maintained even if the length of l is shortened. I understand.

For example, the external cavity length l when dλe/dT = 0.5 Å/°C is
If reflective members of 65 μm and 100 μm are used, the length can be shortened to 40 μm.

On the other hand, the external resonator length dependence (calculated value) of ΔT with the length of the reflecting member as a parameter is as shown in FIG. According to this, the length of the reflective member is 300μm,
When l=65 μm, ΔT=26°C, but if the reflective member is shortened to 100 μm, ΔT=42°C, which widens the temperature range in which mode hops do not occur.

<Examples> The present invention will be described in detail below based on examples shown in the drawings. Note that this invention is not limited to this.

FIG. 2 is a perspective view of an external cavity type semiconductor laser device showing an embodiment of the present invention, in which 1 is a VSIS type semiconductor laser having an active layer of AlGaAs on a GaAs substrate, 1a is one of the emission end faces; Reference numeral 2 denotes a reflective member made of a GaAs chip, and the surface 2a facing 1a has a higher reflectance (90% or more) than the dielectric coating. 3 is a mounting member (Cu heat sink) on which the semiconductor laser 1 and GaAs chip 2 are installed, 4 is an Al
This is the lead wire. 2L is the resonator length of the semiconductor laser, 2M is the length of the reflecting member, and l is the semiconductor laser 1.
is the distance (external resonator length) between the emission end surface 1a and the reflection surface 2a of the reflection member 2.

In this configuration, the cavity length of the semiconductor laser 2L = 250 μm, the length of the GaAs chip 2M =
Each was mounted on a Cu heat sink with an external cavity length of 150 μm and an external cavity length of 45 μm, and the temperature dependence of the oscillation wavelength was measured. The measurement results are shown in Figure 3. This external cavity semiconductor laser does not generate any mode hops in the range of ΔT = 35°C, and can maintain a specific oscillation mode over a 9°C wider temperature range than when 2M = 300 μm (ΔT = 26°C). It maintained stable oscillation.

The cavity length of a semiconductor laser is difficult to shorten because it is necessary to ensure bonding strength on the heat sink in addition to ease of handling the laser chip, and in practice it is 200 μm or more, but GaAs chips do not have lead bonding. Therefore, there is no need to consider bonding strength, and there are no mounting problems even if the length is 100 μm or less. As in the above example, it is effective to shorten the length of the GaAs chip in order to widen ΔT.In the above semiconductor laser device, only the external resonator is controlled, so ΔT can be increased while maintaining the good characteristics of the semiconductor laser itself. Can be expanded.

<Effects of the Invention> As detailed above, according to the present invention, it is possible to control the mode hop of the oscillation mode with respect to temperature changes over a wide temperature range, and it is possible to obtain a semiconductor laser device with stable operating characteristics.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram for explaining the basic principle of the present invention. FIG. 2 is a perspective view of a semiconductor laser device showing an embodiment of the present invention. FIG. 3 is a characteristic diagram showing the temperature characteristics of the oscillation wavelength of the semiconductor laser device shown in FIG. FIG. 4 is an explanatory diagram illustrating the relationship between the external resonator length and the temperature change rate of the external mode using calculated values using the length of the reflecting member as a parameter. FIG. 5 is an explanatory diagram showing the relationship between the external resonator length and ΔT using calculated values using the length of the reflecting member as a parameter. FIG. 6 is an explanatory diagram showing the selectivity of the oscillation mode of a conventional semiconductor laser. FIG. 7 is an explanatory diagram showing the selectivity of the oscillation mode of the SEC semiconductor laser. FIG. 8 is a characteristic diagram showing the temperature characteristics of the oscillation wavelength of a conventional semiconductor laser device. Figure 9 a, b,
c is a characteristic diagram showing various aspects of the temperature characteristics of the oscillation wavelength in a general SEC semiconductor laser. 1...Semiconductor laser element, 2...Reflection member, 3
...Placement member, 4...Lead wire.

Claims (1)

[Scope of Claims] 1. A semiconductor laser element, and a laser beam emitted from one emission end face of the semiconductor laser element is returned to the semiconductor laser element, and an external resonator is formed between the semiconductor laser element and the semiconductor laser element. and a reflecting member having a reflecting surface are respectively mounted and fixed on a mounting member, and the oscillation mode of the laser is determined by three factors: the gain distribution and axial mode of the semiconductor laser element, and the wavelength selectivity of the external resonator. In an external cavity type semiconductor laser device in which the conditions of the elements are set so as not to generate any mode hops in the temperature range ΔT, the length of the reflecting member in the direction parallel to the laser resonance direction is the same as that of the semiconductor laser element. and the temperature range ΔΤ
1. An external cavity type semiconductor laser device, characterized in that the external cavity length is shortened in the direction of enlarging the length of the external cavity. 2. The external cavity type semiconductor laser device according to claim 1, wherein the material of the reflecting member is the same as the material of the substrate of the semiconductor laser element. 3. The external cavity type semiconductor laser device according to claim 2, wherein the length of the reflecting member in the direction parallel to the laser resonance direction is 250 μm or less.