JPH0332090A - Variable wavelength laser - Google Patents

Abstract

PURPOSE:To vary a wavelength within a wide range by connecting a semiconductor laser directly to a reflector comprising a low-loss waveguide and controlling a diffraction grating part and a phase modulating part by utilizing a wavelength selectivity of the reflector. CONSTITUTION:A distribution Bragg reflector 2 comprises a waveguide 21 which is formed separately of a low-loss material compared to an active layer of a semiconductor laser 1. In the reflector 2, a diffraction grating part comprising diffraction gratings 22 is arranged on said waveguide 21 and a phase modulating part is arranged between said diffraction grating part and the semiconductor 1. Then, control electrode 25 and 26 and an earthing electrode 27 control the equivalent refractive indexes of said phase modulating part and diffraction grating part and they can vary the wavelength within a wide range continuously by utilizing a wavelength selectivity of the reflector 2. Additionally, the semiconductor laser 1 and the reflector 2 are attached to one base 3 and are connected directly, so that it enables a mechanical stability and a reduction in size.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used as a light source for coherent optical communication or coherent optical measurement. In particular, distributed Bragg reflectors (D B
RS distributed bragg refl
This invention relates to an external cavity type tunable wavelength laser using an external cavity type tunable laser.

〔overview〕

The present invention provides a tunable wavelength laser equipped with a distributed Bragg reflector as an external resonator, in which the waveguide of the distributed Bragg reflector is formed of a material with lower loss than the active layer. This is to narrow the spectral line width of laser oscillation by increasing the wavelength, and to expand the wavelength tuning range.

[Conventional technology]

Semiconductor lasers with a narrow spectral linewidth of IMHz or less and a variable wavelength are attracting attention as light sources for coherent optical communication or coherent optical measurement. In order to narrow the spectral linewidth, basically, (1-1) increase the Q value of the laser resonator by increasing the resonator length by attaching an external mirror or optical fiber, (1 -2) In semiconductor lasers such as distributed feedback lasers whose oscillation frequency can be controlled by injection current, a method is used to restore the oscillation frequency to its original value by detecting fluctuations in the oscillation frequency and applying negative feedback to the injection current. .

In addition, in order to make the wavelength variable, (2-1) use a diffraction grating as an external mirror and rotate it; (2-2) provide a quantum well distributed feedback laser with multiple independent electrodes to adjust the diffraction wavelength and Methods such as controlling the phase are known.

[Problem to be solved by the invention]

However, in the methods (1-1) and (2-1) above, the positional relationship between the laser oscillation element and the external mirror or optical fiber is mechanically maintained, so it is not susceptible to mechanical vibrations or other disturbances. There are weak drawbacks. Furthermore, when the wavelength is made variable, it is difficult to adjust the phase between the laser oscillation element and the external mirror.

In order to solve this problem, a device in which a laser oscillation element and a diffraction grating are formed monolithically is also known. In this case, to make the wavelength variable, a method is used in which an electric field or current is applied to the diffraction grating to change the effective refractive index. However, due to the large loss of the optical waveguide, it was not possible to increase the resonator length, and it was not possible to obtain a large Q value.

Furthermore, with the methods (1-2), (2-1), and (2-2), so far no method has been obtained that fully satisfies both the narrowing of the spectral line width and the wavelength tuning range. Not yet.

An object of the present invention is to solve the above problems and provide a tunable wavelength laser that has a narrow spectral linewidth, a wide wavelength tunable range, and operates stably.

[Means to solve the problem]

In the tunable wavelength laser of the present invention, a distributed Bragg reflector whose waveguide is formed of a material with lower loss than the active layer of a semiconductor laser is formed separately from the semiconductor laser, and the distributed Bragg reflector is formed separately from the semiconductor laser. It is characterized by being attached to a single stand along with the laser.

A distributed Bragg reflector consists of a diffraction grating section in which a diffraction grating is formed on a waveguide, a phase modulation section provided between the diffraction grating section and the semiconductor laser, and each of the diffraction grating section and the phase modulation section. It is desirable to include an electrode for controlling the equipolarized refractive index.

The active layer of a semiconductor laser is usually ALGaAs or I
It is formed of nGaAsP. As a low loss material for these materials, Ti-diffused LiNb0. Alternatively, proton exchange 1iTaos can be used.

It is desirable to provide an antireflection coating on the end face of the semiconductor laser, particularly on the end face facing the distributed Bragg reflector.

[For production]

Since the waveguide of the distributed Bragg reflector has low loss,
A large Q value can be obtained even with a relatively small reflector, and the spectral line width of laser oscillation can be narrowed. Further, by utilizing the wavelength selectivity of the distributed Bragg reflector, a semiconductor laser can be operated in a single longitudinal mode. Furthermore, by controlling the refractive index of the diffraction grating section and the phase modulation section using the electro-optic effect of the waveguide, the wavelength can be changed continuously over a wide range.

〔Example〕

FIG. 1 shows a plan view of a tunable wavelength laser according to an embodiment of the present invention,
FIG. 2 shows its sectional view.

This variable wavelength laser includes a Fabry-Perot type semiconductor laser 1 and a distributed Bragg reflector 2 that reflects light of a specific wavelength included in the light emitted from the semiconductor laser 1.

Here, the feature of this embodiment is that the liquid guiding path 21 of the distributed Bragg reflector 2 is formed separately from the semiconductor laser 1 by a material having a lower loss than the active layer of the semiconductor laser I. This is because the distributed Bragg reflector 2 and the semiconductor laser 1 are mounted on one stand 3.

One end face 11 of the semiconductor laser l is an open face, and the other end face 2 is provided with an antireflection film. The distributed Bragg reflector 2 is placed on the side of the end face 12 provided with the antireflection film. The semiconductor laser 1 and the distributed Bragg reflector 2 are fixed on a table 3 made of invar or ceramic.

The distributed Bragg reflector 2 has a diffraction grating 22 on the waveguide 2
a diffraction grating portion formed with a phase modulation portion, a phase modulation portion provided between the diffraction grating portion and the semiconductor laser, and control electrodes 25 and 26 for controlling the equivalent refractive index of each of the phase modulation portion and the diffraction grating portion. and a ground electrode 27. The end faces 23, 24 of the distributed Bragg reflector 2 are each provided with an antireflection coating. Control electrodes 25,26 are connected to terminals 28.
29 is connected.

The active layer of the semiconductor laser I is made of AjGaAs or InG.
The waveguide 21 of the distributed Bragg reflector 2 is formed of an aAsP-based material, and is formed of lithium niobate L+Nb0a with titanium Ti diffused therein or proton-exchanged lithium tantalate LiTaO3.

1NbO, or LiTa0. As a method for forming the diffraction grating 22 in the waveguide 21, for example, reactive ion etching can be used. In the case of reactive ion etching, LiTa0. is easier to process.

Alternatively, the diffraction grating 22 can be obtained by periodically depositing silicon oxide (SiO2), silicon nitride (SiN), photoresist, or other dielectric material on the surface of the waveguide 21. Furthermore, a combination of a grating formed by etching the waveguide 21 and a dielectric deposited on the surface can be used.

The operating conditions, spectral linewidth, conditions for single mode oscillation, wavelength variable characteristics, phase modulation characteristics, and phase control by the phase modulation section of this embodiment will be explained in order.

To find the operating conditions, L: resonator length of semiconductor laser 1 alone g: gain coefficient αL of semiconductor laser 1: loss coefficient β of semiconductor laser 1: propagation constant ni of semiconductor laser 1: equipolarized refraction of semiconductor laser 1 rate αw: loss coefficient of the waveguide 21 8w: propagation constant N of the waveguide 21: equivalent refractive index L of the waveguide 21: waveguide length of the phase modulation section, that is, equivalent distance from the end surface 23 of the waveguide 21 to the diffraction grating 22 Distance 12 to the reflecting surface: Waveguide length of the diffraction grating section, that is, the diffraction grating 22
Distance e3 from the equivalent reflecting surface to the end surface 24: Distance ψ between the semiconductor laser 1 and the distributed Bragg reflector 2 7: Electric field reflectance of the end surface 11 h: Electric field reflectance of the end surface 12 1 Co: End surface 23 electric field reflectance of the semiconductor laser 1 and the waveguide 21 The coupling efficiency of the diffraction grating 22 is assumed to be the equivalent reflection surface of the diffraction grating 22.
This is the starting surface of 2. The above parameters are shown in FIG.

Here, in order to facilitate the analysis of the operation, the electric field reflectance q,
q and interval f! 3 can be ignored. The electric field reflectance -, H, and n can be reduced to negligible values by, for example, providing an antireflection film on the end face 23 and cutting the end face 24 diagonally (several degrees).

At this time, the oscillation conditions and gain conditions of the single semiconductor laser l are -IT'; nexp[-J2βL+(g-αt)
L]-1(1). Furthermore, assuming that the end face 11 of the semiconductor laser 1 and the equivalent reflection surface of the diffraction grating 22 constitute an optical resonator, the oscillation conditions and gain conditions of the external cavity laser including this optical resonator are determined as follows. 42ψ7exp[-J(2
βL+2βwi++φ)+(g-αt)L-α,f, ] =l ・−・・−(3)・
・ ・ (4) becomes.

The power P generated in the laser medium in the semiconductor laser 1 is −(5). However, ηi: internal quantum efficiency h, blank constant ν: oscillation frequency e: electronic charge ■, J: injection current, injection current density I*hSJth: L threshold current, threshold current density W:
This is the active layer width of the semiconductor laser 1.

Empirically, the threshold gain is It is expressed as however, β. , Jo: constant d: Thickness of active layer It is.

In the gain conditions of equations (2) and (4), the true gain needs to be expressed as Fg using the confinement coefficient F to the active layer. Therefore, the threshold currents of the semiconductor laser 1 alone and the external cavity laser are (2), (4)
, using equation (6), (7) ■.

(External resonator type LD) ・・・・・・(8) becomes.

Therefore, in order for the oscillation of the external cavity laser to be more dominant than that of the semiconductor laser 1 alone, it is sufficient that 1th (LD alone) > Ith (LD alone). That is, tnRl>LnR2-1nco2+ 211+αw (9
) should be satisfied.

FIG. 4 shows an example of the relationship between reflectance R9 and coupling efficiency C0 to satisfy equation (9). In this diagram, I! , =
10mm, a, = 0.0115 nepa/mm,
The reflectance R2 is shown as a parameter. As shown in this figure, if R2 <1%, Co = 30%, R4 =
It can be seen that about 20% satisfies equation (9).

Practically, it is possible to reduce the reflectance R2 to 0.1% or less by providing a reflective coating. Further, waveguide loss is usually expressed in dB/cm, and a loss of 1 dB/cm corresponds to α = 0.01151 n/mm.

Next, the spectral linewidth will be explained.

Semiconductor laser oscillation spectrum width (full width at half maximum) Δν. .

is the Shalow-Towns formula considering the linear convergence increase coefficient α (α parameter), πhνn, , (Δνc)2 Δν. sc = (1+α2)
...---00 is given. However, n□: Spontaneous emission coefficient, usually 1 to 2ΔνC: Spectral width of the optical resonator δ: λ. : A : It is.

Frequency detuning (=2πN (1/λ-17λo)) Bragg wavelength (=2NA) Pitch of the diffraction grating Therefore, the power reflectance is -00. The coupling constant is determined by the shape of the diffraction grating and the structure of the waveguide. For example, increasing the depth of the grating increases the coupling constant.

FIG. 7 shows the dependence of the distributed Bragg reflection characteristics on . This figure shows the center wavelength (Bragg wavelength) λo-1, 55 μm5
Let j72=1cm and λ. The reflectance R7 is shown for the detuning Δλ from . Coupling coefficient=Q, 5 cm-'lc+
For yr' and 2cm-', the half width at half maximum is 0.
26 people, 0.32 people, and 0.48 people, and the reflectance R7 at the center wavelength is 21%, 58%, and 93%, respectively.

On the other hand, the longitudinal mode spacing Δν(
Since is λ2, single mode operation is fully possible by setting λ2 to about 1 cm−'.

Next, the wavelength variable characteristics will be explained.

Bragg wavelength λ of a distributed Bragg reflector. is λO=A/
2N...-■ Therefore, in order to change the wavelength, it is sufficient to change the equipolarized refractive index N. λ. In order to change N by ±2.5 nm, change N by ±3.
5 The distance ξ between the control electrode 26 and the ground electrode 27 shown in the figure is an electric field correction coefficient. d, = 10 μm, ξ = 0.4, ΔN = 3.5
If xlO-', then V=550V.

Next, phase control by the phase modulation section will be explained.

From the oscillation condition in (3), the phase condition is 2βL+2β, (1+φ=2πrn (m=0.1, 2...) −・ −・−・・
(2), and the phase change in one round trip should be an integral multiple of 2π. That is, like the diffraction grating section, the equal polarized refractive index of the phase modulation section may be controlled by voltage. The amount of phase change Δψ due to voltage in the phase modulation section is Δψ = (2π/λ)・2ΔN
-1゜=(2π/λ) N' r33 (V/dw)
It becomes Il+ξ. For example, to change by Δψ=π, 11=lQmmSdw=10 μm, ξ=0.4, and V=6V. However, d at this time is the distance between the control electrode 25 and the ground electrode 27. This interval is
In practice, the distance is equal to the distance between the control electrode 26 and the ground electrode 27.

〔Effect of the invention〕

As explained above, the tunable wavelength laser of the present invention has a narrow spectral linewidth of 100 kHz or less, oscillates in a single mode due to the wavelength selectivity of the distributed Bragg reflector, and has a uniform refractive index of the phase control section and the diffraction grating section. The wavelength can be changed by control. Furthermore, since the semiconductor laser and the external reflector are directly coupled without intervening fibers, lenses, etc., there is an effect that the device is mechanically stable and can be miniaturized.

[Brief explanation of drawings]

FIG. 1 is a plan view of a tunable wavelength laser according to an embodiment of the present invention. Figure 2 is a sectional view. FIG. 3 is a diagram showing parameters for determining operating conditions. FIG. 4 is a diagram showing an example of the relationship between reflectance R9 and overall efficiency co. Figure 5 shows the oscillation spectrum width Δν. . FIG. 3 is a diagram showing an example of the relationship between the waveguide length 11 of the phase modulation section and the waveguide length 11 of the phase modulation section. FIG. 6 shows the oscillation spectrum width Δν. , C and the coupling efficiency co. FIG. 7 is a diagram showing the dependence of distributed Bragg reflection characteristics on . ■... Semiconductor laser, 2... Distributed Bragg reflector,
3...stand, 11.12.23.24...end face, 21
...Waveguide, 22...Diffraction grating, 25.26...
Control electrode, 27... Ground electrode, 28.29... Terminal.

Claims (1)

[Claims] 1. A tunable wavelength laser comprising a Fabry-Perot semiconductor laser and a distributed Bragg reflector that reflects light of a specific wavelength included in the light emitted from the semiconductor laser, the distributed Bragg reflection described above. The variable device is characterized in that the waveguide is formed separately from the semiconductor laser using a material with a lower loss than the active layer of the semiconductor laser, and is mounted on a single stand together with the semiconductor laser. wavelength laser. 2. The distributed Bragg reflector includes a diffraction grating section in which a diffraction grating is formed on a waveguide, a phase modulation section provided between the diffraction grating section and the semiconductor laser, and the above diffraction grating section and the phase modulation section. 2. The tunable wavelength laser according to claim 1, further comprising an electrode for controlling the equivalent refractive index of each of the portions. 3. The tunable wavelength laser according to claim 1, wherein the low loss material comprises Ti-diffused LiNbO_3. 4. The tunable wavelength laser according to claim 1, wherein the low loss material comprises proton-exchanged LiTaO_3.