JPH0344993A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To obtain a semiconductor device which can be stable with moisture change by forming a reflecting mirror of a fabricated bellow resonator with a dielectric multilayer film whose filling density is specified. CONSTITUTION:The emission light of a semiconductor laser is partially received by way of a fabricated bellow resonator formed by assembling a reflecting mirror with a dielectric multilayer film. In a semiconductor laser device which stabilizes the wavelength of emission light by controlling the oscillation of the semiconductor laser based on a received light signal, the filling density of the reflecting mirror used for the fabricated bellow resonator is specified to exceed 0.98. This construction makes it possible to increase the filling density, minimize refracting power change due to moisture and stabilize the performance against moisture. It is, therefore, possible to stabilize the wavelength characteristics of the resonator and hence stabilize the wavelength of the semiconductor laser.

Description

DETAILED DESCRIPTION OF THE INVENTION [Objective of the Invention (Industrial Application Field) The present invention relates to a semiconductor laser device used, for example, in optical communication (coherent communication, etc.).

(Prior art) Conventionally, the second type of semiconductor laser device used for optical communication is
There is one shown in the figure. In FIG. 2, 11 is a semiconductor laser whose oscillation wavelength (oscillation frequency) can be controlled. The light S1 emitted from the left side of the semiconductor laser 11 in the figure is transmitted to the lens 12. The light is focused on the input end of the communication optical fiber 14 via Is.

Also, the light S emitted from the right side in the figure of the semiconductor laser 11
2 is converted into parallel light by the lens 15, and then split into two directions by the beam splitter i6.

One of the branched lights S3 is focused on a photodiode 18 via a lens 17. The other light S4 is incident on the Fabry-Perot resonator 19.

This Fabry-Perot resonator 19 has a pair of reflecting mirrors 191.
, 192 facing each other on the optical axis and separated by a distance ML, and the light transmitted through this resonator 19 is
The free spectral spacing λ2/2nL (
λ: wavelength, n: refractive index in Fabry-Perot cavity 19)
The light has the characteristic of repeating as a period, and the lens 20
The light is focused on the photodiode 21 via the.

Therefore, the light reception signal of the photodiode 21 is transmitted to the resonator 19.
changes depending on the frequency change received by the transmitted light. Therefore, by measuring the ratio of the respective light receiving outputs of the photodiodes 18 and 21, the oscillation wavelength λ of the semiconductor laser ti can be determined. Generally, in order to control the oscillation wavelength of this semiconductor laser 11, the oscillation wavelength of the semiconductor laser H is detected from the ratio of the output values of the photodiodes 18 and 21, and the temperature of the semiconductor laser L1 or the injection current is changed. ,
Control to desired wavelength.

Incidentally, the reflecting mirror of a conventional Fabry-Perot resonator used in such a device is formed of a dielectric multilayer film with a packing density of about 0.9. It is known that the wavelength characteristics of a dielectric multilayer film with a packing density of 0.9 will shift depending on the humidity in the air, but conventionally this level is sufficient for a reflector used in a Fabry-Perot resonator for the following reasons. It is considered.

■The characteristics of the Fabry-Perot resonator are given by α-4πnLcosθ/λ F-π, /'i/(1-R). Here, θ is the forerunner angle to the Fabry-Perot resonator, F is the finesse of the Fabry-Perot resonator, and R is the reflectance of the reflector. From this, the amount that affects the wavelength axis of the characteristic is only nL cos θ, and does not depend on the wavelength shift of the multilayer film.

■The reflectance characteristics of the dielectric multilayer film used in the Fabry-Perot resonator vary gradually with wavelength. Therefore, even if a wavelength shift of the multilayer film occurs due to a change in humidity, the change in reflectance is extremely small and does not pose a problem.

However, when we verified the above idea through highly accurate experiments, we found that the dielectric multilayer film that forms the reflecting mirror is actually affected by humidity, and its refractive index changes by 5 to 6%. This changed the phase of the reflected wave and shifted the wavelength characteristics of the resonator. If the wavelength characteristics of the Fabry-Perot resonator shift in this way, the oscillation wavelength of the semiconductor laser that uses the resonator as a wavelength reference will also shift, making it impossible to obtain stable operation against humidity. Generally, humidity is also related to temperature, so devices with poor humidity stability tend to have poor temperature stability as well.

(Problems to be Solved by the Invention) As described above, in conventional semiconductor laser devices, the dielectric multilayer film used for the reflecting mirror of the Fabry-Perot cavity has a packing density of about 0.9, and the refractive index changes due to temperature changes. As a result, the phase of the reflected wave changes, shifting the wavelength characteristics of the resonator, making it impossible to obtain stable operation with respect to temperature.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a semiconductor laser device that has good stability against changes in humidity.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, a semiconductor laser device according to the present invention is configured by combining a reflecting mirror made of a dielectric multilayer film to partially reflect the emitted light of the semiconductor laser. In a device that receives light through a Fabry-Perot resonator and stabilizes the wavelength of the emitted light by controlling the oscillation of the semiconductor laser based on the received light signal, the packing density of the reflecting mirror used in the Fabry-Perot resonator is set to 0. It is characterized by being .98 or higher.

(Function) In the semiconductor laser device having the above configuration, by forming the reflecting mirror with a dielectric multilayer film having a packing density of 0.98 or more,
By increasing the packing density and reducing the change in refractive index due to temperature,
Minimizes the phase change of reflected waves, suppresses shifts in resonator wavelength characteristics, and stabilizes operation against humidity. As a result, the wavelength characteristics of the resonator can be stabilized, and the wavelength of the semiconductor laser can be stabilized.

(Embodiment) An embodiment of the present invention will be described below with reference to FIG.

Figure 1 shows the aforementioned Fabry-Perot resonator as a simple model. Here, dielectric monomers with a refractive index nH and an optical length λ/4 are placed at both ends of a material A with a refractive index n and a length L. It is assumed that a layer film B is formed.

The dielectric single layer film B is porous, and its refractive index n H
is given by the following equation.

Here, n3 is the refractive index of the thin film material, ny is the refractive index of the void (-1), nw is the refractive index of the adsorbent (-1, 33),
p is the packing density, and f is the proportion of voids occupied by the adsorbed substance. As is clear from equation (1), when the humidity changes, f changes and nH changes. Now, the thickness of this single layer film is expressed as: D1 wavelength is λ, refractive index n 14 at f-0 is n Ho, and g - λ/4 n H.

Make it so that Here, in FIG. 1, the reflected light a
] Consider +a2. a1□ a2ζ is the complex amplitude of the reflected light, and can be regarded as 1a11 et 1a・-〇"-°゛=a nH0nL. Under this condition, al+"2 is found.

a. a e 1(iv++g)
---('1)j(-dragon+2j+
+HjX2/λ) a 2-ae w a e l Im++z(1+a+(However, nn/
noo-1+δ) ... (3) Therefore, from equations (2) and (3), a 1 + a 2
．． II a e l (m i 4#) (1+
e l *'), it can be seen that when the refractive index nH of the single layer film changes, the phase of the reflected wave changes by about πδ/2 (1πδ1 (when 1)).

In a Fabry-Perot resonator, a shift in wavelength characteristics due to a phase shift from a reflecting mirror is a more important problem than a 1% change in the reflection coefficient a. This invention considers the wavelength shift caused by such a phase shift as a problem. A wavelength shift is established. Here, h is the effective resonator length, h=*nL ah-(λ/2π) (π δ/2) − λ δ/4.

If you apply the numerical value specifically, it is hm2mm.

If λ-1.5μ and δ-0.05, aλ is 0.14
It will be entered. This is the free space spectral interval (λ2/2h
) corresponds to a 2.5% variation in

The above is about a single layer film, but if it becomes a multilayer film, the amount of phase change increases almost in proportion to the number of layers, and the wavelength shift also increases. On the other hand, multilayer films tend to suppress fluctuations in the amount of reflection due to humidity, and wavelength shift becomes a bigger problem.

Therefore, in the present invention, the reflecting mirror of the Fabry-Perot resonator is formed of a dielectric multilayer film having a packing density of 0.98 or more.

In other words, as can be seen from equation (1), the higher the packing density, the smaller the change in refractive index due to temperature. This suppresses the phase change of the reflected wave from the reflecting mirror. In particular, when the resonator length of the Fabry-Perot resonator is short, the amount of wavelength shift increases as shown by equation (4), so this is particularly effective for resonator lengths of about 5 mm or less. Therefore, the wavelength characteristics of the Fabry-Perot resonator become stable, and the Fabry-Perot resonator becomes stable.
The wavelength of a semiconductor laser device incorporating a resonator also becomes stable. Since temperature is also a function of temperature, the above configuration also improves stability with respect to temperature. Densely packed dielectric multilayer films can be formed using improved electron beam evaporation equipment, sputtering equipment, and the like.

Note that this invention is not limited to the above embodiments,
Needless to say, in addition to the Fabry-Perot resonator shown in FIG. 3, the present invention can also be applied to a device in which a dielectric multilayer film is formed on both ends of a single parallel plate.

Furthermore, there are various methods, such as obtaining an error signal by performing synchronous detection with a wavelength detection low frequency signal using a Fabry-Perot resonator. The present invention is effective for any wavelength detection using these Fabry-Perot resonators.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide a semiconductor laser device that is highly stable against changes in humidity.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a model of a Fabry-Perot resonator, which is a feature of an embodiment of the semiconductor laser device according to the present invention, FIG. 2 is a perspective view showing the general configuration of the semiconductor laser device, and FIG. The figure is a characteristic diagram showing the wavelength characteristics of the Fabry-Perot resonator. 11... Semiconductor laser, 12.13.15.17.2
0... Lens, L4... Optical fiber for communication, 1B.
...Beam splitter, 18.21...Photodiode, 19...Fabry-Perot resonator, 191, 1
92... Reflecting mirror, A... Transmissive material, B... Dielectric single layer film.

Claims (1)

[Claims] A part of the emitted light from the semiconductor laser is received through a Fabry-Perot resonator composed of a combination of reflecting mirrors made of a dielectric multilayer film, and the oscillation of the semiconductor laser is controlled based on the received light signal to change the wavelength of the emitted light. A semiconductor laser device that is stabilized, characterized in that the packing density of a reflecting mirror used in the Fabry-Perot cavity is 0.98 or more.