JPH08340152A - Semiconductor pulsed laser - Google Patents

Abstract

PURPOSE: To provide a semiconductor pulsed laser, which does not require to take the accuracy of a length for cutting out an element into consideration and can change the repetitive frequency. CONSTITUTION: A saturable absorption region 21 performs an action to make a passive mode-locking cause by applying a reverse bias voltage from a boltage source 31 through a P-side ohmic electrode 8. A gain region 22 functions as a gain medium by applying a current from a current source 32 through a P-side ohmic electrode 9 and performs 'an optical amplitude action, which is required for the generation of a laser beam and a laser oscillation. A Bragg reflecting mirror region 23 is provided so as to function as a wavelength filter incorporated in a resonator by the function of a diffraction grating 7. Therefore, the region 23 performs the control of an oscillation wavelength and the role of the limitation of an oscillation wavelength range and inhibits the effect of the dispersion of the refractive index of the laser beam to have a function to easily make the passive mode-locking cause. The region 23 is split into four and the four regions are formed so as to be able to perform independently an injection of currents from the source 32 and current sources 33 to 36 in the respective regions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor pulse laser device for generating an ultrashort optical pulse train used for optical measurement and optical communication.

[0002]

2. Description of the Related Art Conventionally, techniques in such a field include:
For example, as document (1), “P. B. Hansen,
G. Raybon, U.S.A. Koren, B.A. I. Mill
er, M .; G. Young, M .; Chien, C.I. A.
Burrus, and R.M. C. Alferness,
"5.5-mm Long InGaAsP Mono
lithic Extended-CavityLas
er with an Integrated Bra
gg-Reflector for Active M
ode-Locking "IEEE Photon.T
ech. Lett. VoL4, No. 3, P.I. 215-
217 (1992) ”Document (2) was described in“ Semiconductor Laser and Optical Integrated Circuit, Yasuharu Suematsu, Ohmsha, Ltd., pages 325-339 ”.

The semiconductor pulse laser device described in the above-mentioned document (1) generates an optical pulse train having a repetition that matches the orbital frequency of the element by the active mode locking method. In this document, on the same InP substrate, a gain region for generating and amplifying light, a passive waveguide having a composition having a bandgap wavelength shorter than that of the gain region, a bandgap composition having the same bandgap composition as the passive waveguide, and a diffraction grating This paper describes an active mode-locking method using an integrated device with the Bragg reflector region.

The length of each region is 700 μ in the gain region.
m, the passive waveguide is 4200 μm, and the Bragg reflector area is 600 μm. The total length of each area is 5500 μm
Is. Circular frequency of the element in the gain region (8.1 GHz)
An optical pulse train is generated by the active mode-locking method by sending a modulation current that coincides with. In this document, the pulse width is 20
We have succeeded in generating an optical pulse train with a conversion limit of ps.

The repetition frequency of the optical pulse generated by the element, 8.1 GHz (120 ps), corresponds to the time for the optical pulse to make one round trip in the resonator formed by the element.
In this case, when calculating the spatial length of the optical pulse actually reciprocating, the length of the Bragg reflector is replaced with the effective light penetration length, and then the total of each region may be obtained.

An example of calculating the penetration depth of light into the Bragg reflector is disclosed in the above-mentioned document (2). Penetration length means the light that has entered the Bragg reflector is 1 / e of the incident light intensity.
The Bragg mirror is equivalent to placing the mirror at this position. In the case of the above document, the Bragg reflector region length of 600 μm is equivalently equal to the length of 200 μm, and the repetition frequency is determined to correspond to 5100 μm in which the gain region is 700 μm and the passive waveguide is 4200 μm.

[0007]

However, in the conventional example disclosed in the above document, the repetition frequency is determined by the length of the manufactured element, and in order to obtain the repetition frequency actually required for application. Had to request the precision of the length of cutting out the element so as to satisfy the precision of the repetition frequency.

Therefore, the present invention eliminates the above problems,
An object of the present invention is to provide a semiconductor pulse laser device in which the repetition frequency can be changed without having to consider the accuracy of the length of cutting out the element.

[0009]

In order to achieve the above object, the present invention provides (1) a semiconductor pulse in which a saturable absorption region, a gain region, and a Bragg reflector region which are semiconductor waveguides having the same composition are integrated. In the laser device, by dividing the Bragg reflector region and changing the refractive index and the absorption coefficient between these regions, the penetration length of incident light is changed and the repetition frequency of the mode-locked laser can be changed. It is a thing.

(2) In the semiconductor pulse laser device described in (1) above, the Bragg reflector area is divided into four first to fourth reflector areas, and all of the first to fourth Bragg reflector areas are formed. Does not inject current into the Bragg reflector region and causes laser oscillation at an oscillation wavelength determined by the Bragg wavelength λb in these Bragg reflector regions. Next, for the second to fourth Bragg reflector regions, By performing tuning, the second wavelength with respect to the oscillation wavelength λb of the laser can be adjusted.
To the fourth Bragg reflector region.

(3) In the semiconductor pulse laser device according to the above (1) or (2), the composition of the waveguide forming the Bragg reflector region should be equal to or longer than the oscillation wavelength. It was done. (4) In the semiconductor pulse laser device according to (1) or (2), the composition of the waveguide forming the Bragg reflector region is a bulk or quantum well structure having a wavelength shorter than the oscillation wavelength by about 50 nm. It is the one.

[0012]

In the semiconductor pulse laser device of the present invention, as described above, the saturable absorption region, the gain region, and the Bragg reflector region are integrated on the same substrate, and each region has a waveguide of the same composition. To have. It is assumed that each region has sufficient electrical isolation and no electrical interference effect. Further, the waveguide serving as the saturable absorption region has an electrode for applying a reverse bias voltage from the outside, and also has a power supply for applying the reverse bias voltage.

Further, each of the divided Bragg reflector regions has an electrode for current injection or forward bias from the outside, and a power supply for applying current injection or reverse bias. are doing. The Bragg reflector region has a diffraction grating having a pitch (Λ1) corresponding to the oscillation wavelength and a region (L1) where the diffraction grating is formed.
And a sampled grating having a double period having a region (L2) not formed as one period (Λ2).

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a side view schematically showing the structure of a semiconductor pulse laser device according to an embodiment of the present invention. FIG.
In the saturable absorption region 21, the p-side ohmic electrode 8
A reverse bias voltage is applied from the voltage source 31 through the through, thereby causing an action of causing passive mode locking. Also,
The gain region 22 acts as a gain medium by injecting a current from the current source 32 through the p-side ohmic electrode 9, and has a light amplifying action required for light generation and laser oscillation.

Further, the Bragg reflector region 23 functions as a wavelength filter incorporated in the resonator due to the function of the diffraction grating 7. Therefore, it has a role of controlling the oscillation wavelength and limiting the oscillation wavelength region, and has a function of suppressing the influence of refractive index dispersion and facilitating passive mode locking.
In FIG. 1, the Gragg reflecting mirror region 23 is divided into four parts, and currents can be independently injected from the current sources 33 to 36.

The calculation result for the light penetration length of the Bragg reflector having a uniform diffraction grating is as described in the above-mentioned reference (2). This penetration length is due to the waveguide loss and diffraction of the diffraction grating. It depends on the coupling coefficient of the grating and also largely on the wavelength of the incident light. In the calculation example of the above-mentioned document (2), the wavelength of the incident light is assumed to be a wavelength (Bragg wavelength λb) that matches the pitch of the diffraction grating. For example, the coupling coefficient of the diffraction grating normally used in such a structure is , 20 cm ⁻¹ or more due to the accuracy of the manufacturing process of the diffraction grating, but in this case, the penetration length becomes longer according to the length of the Bragg reflector, but there is an upper limit and the maximum is 200 μm.
It turns out that it stays around m.

Further, when the absorption loss of the waveguide is large,
Penetration length is infinitely small. Now, consider a method of increasing the maximum penetration depth in order to greatly change the repetition frequency of the mode-locked laser. The first of this method can be realized by using a sampled grating. The penetration depth of light to the sampled grating is (L1 + L2) / the penetration length when there is a uniform grating, where L1 is the region where the diffraction grating is formed and L2 is the region where the diffraction grating is not formed.
Since it can be increased to L1 times, by setting this ratio to a desired value, it is possible to obtain a penetration length of 200 μm or more. For example, the penetration length for a sampled grating with a total length of 1600 μm, where the region where the diffraction grating is formed is 10 μm as L1 and the region L2 where the diffraction grating is not formed is 70 μm, the total length of the region where the grating is present is 1 μm is 200 μm.
Penetration depth of 80μ for a 200μm grating
It becomes 600 μm which is eight times m.

Next, a method of changing the penetration depth of light will be described. The wavelength dependence of the penetration length is shown in FIG.
Inferred from 1.18. That is, as shown in FIG. 2, when the incident light exactly matches the Bragg wavelength, it receives the strongest reflection (the reflection coefficient R is maximum), but it is completely transmitted in the immediate vicinity (1 to 2 nm). It turns out that there are wavelengths that are Light incident at this wavelength can enter the Bragg reflector without being reflected,
The penetration length will match the length of the Bragg reflector.
In FIG. 2, the wavelength characteristics of the reflection coefficient R and the transmission coefficient T of the distributed reflector having the length L _B are shown, and δ represents the detuning from the Bragg wavelength λb.

As a method (tuning) for controlling the wavelength difference between the Bragg wavelength and the wavelength of the penetrating light, there is a method of utilizing the change in the refractive index using the plasma effect by the current injection into the Bragg reflector region. . The magnitude of the change in the refractive index due to the plasma effect is about 1%, and it is possible to change the wavelength by about 5 nm. In addition, as another method, there is a method in which a current flowing by applying a reverse bias to the Bragg reflector region causes heat generation in the vicinity of the Bragg reflector region and the Bragg wavelength is changed to the long-wave side by this temperature change. .

It is known that the wavelength shift amount obtained by this method can be changed by several nanometers, and it can be said that the method can be carried out. In the present invention, as a specific first example for implementing the method of greatly changing the penetration depth of light, for example, the Bragg reflector region 1600 μm is divided into four, and the Bragg reflector regions 1 to 4 are injected with current. Not done,
A state in which laser oscillation occurs at an oscillation wavelength determined by the Bragg wavelength λb in these Bragg reflector regions is realized. The light penetration length in this case is designed to be, for example, about 600 μm. Next, the Bragg reflector area 2
By tuning 4 to 4, the Bragg reflector regions 2 to 4 for the laser oscillation wavelength λb are set to transmit.

As a result, most of the incident light penetrates to the cleaved end face. When the injection current into the Bragg reflector regions 2 to 4 is significantly increased, the light penetration length can be significantly reduced due to the effect of absorption of light by free electrons. It can be reduced to the region length. According to these methods, in this case, the penetration length is 400 μm to 160 μm.
It can be changed up to 0 μm. This amount of change indicates that the repetition frequency can be controlled up to 35 to 70 GHz in the element in which the total of the active layer region and the saturable absorption region is 800 μm.

In the second example, the composition of the waveguide forming the Bragg reflector region is equal to or longer than the oscillation wavelength. In this case, by injecting current, the medium becomes transparent to the wavelength of incident light, and when not injecting current, absorption occurs. For example, if current is injected into the Bragg reflector region 1 to such an extent that the medium becomes transparent, and no current is injected into the Bragg reflector regions 2 to 4, the effective penetration length will be about 200 μm, and When uniformly injected, the refractive index is changed by adjusting the injection amount to 600 μm, and the Bragg reflector regions 2 to 4 to change the laser oscillation wavelength λb.
It is possible to increase the penetration depth up to 1600 μm by setting the Bragg reflector regions 2 to 4 to be transparent.

In the third example, a bulk or quantum well structure is used in which the composition of the waveguide forming the Bragg reflector region is shorter by about 50 nm than the oscillation wavelength. in this case,
By applying a reverse bias, the refractive index is changed by the Pockels effect in the case of the bulk and by the quantum confinement effect in the case of the quantum well structure, so that the Bragg reflector regions 2 to 4 for the laser oscillation wavelength λb are transmitted. If set to 1, it is possible to increase the penetration length to 1600 μm.

Next, the operation of the device manufactured according to the embodiment of the present invention will be described. In this example,
A semiconductor quantum well structure having a bandgap wavelength of about 1.55 μm was adopted as the waveguide. 1 is an n-side ohmic electrode, 2 is an n-InP clad layer having a thickness of about 150 μm,
3 is an InGaAsP guide layer with a thickness of about 600 Å, 4 is a semiconductor multiple quantum well structure, 5 is an I with a thickness of about 1200 Å
nGaAsP guide layer, 6 is p-In with a thickness of about 2 μm
P clad layers and 8 to 13 are p-side ohmic electrodes.

The diffraction grating 7 has a swing width of about 500Å and a period of 2400Å on the InGaAsP guide layer 5.
It was obtained by forming the sinusoidal unevenness of. The embedding in the lateral direction was a buried hetero structure of p-InP and n-InP. As a method for producing the sampled grating, a uniform grating may be first produced and then partially removed by chemical etching.

The semiconductor multiple quantum well structure 4 has 70 wells.
Å Thick InGaAs (bandgap wavelength 1.67μ
m), the barrier between them is 140 Å thick InGaAsP
(Bandgap wavelength 1.3 μm) to form a five-layer structure. The electrodes 8 to 11 were obtained by once depositing Au on the entire surface and then etching the electrode separation portion by chemical etching. The length of each region is 50 μm in the saturable absorption region 21, and the gain region 2
2 is 800 μm, and the Bragg reflector area 23 is 1600 μm
m. The total length is 2450 μm.

The present invention is not limited to the above embodiments, and various modifications can be made based on the spirit of the present invention, and these modifications are not excluded from the scope of the present invention.

[0028]

As described in detail above, according to the present invention, the following effects can be obtained. (1) According to the invention of claim 1, the saturable absorption region,
In a semiconductor pulse laser device in which a gain region and a Bragg reflector region are integrated, the Bragg reflector region is divided and the penetration length of incident light is changed by changing the refractive index and absorption coefficient between these regions. Therefore, the repetition frequency of the mode-locked laser can be changed significantly.

(2) According to the second aspect of the invention, the Bragg reflector area is divided into four first to fourth reflector areas, and all of the first to fourth Bragg reflector areas are provided. No current injection was performed, and laser oscillation was generated at the oscillation wavelength determined by the Bragg wavelength λb in these Bragg reflector regions, and then tuning was performed for the second to fourth Bragg reflector regions. By doing so, the second to fourth Bragg reflector regions for the oscillation wavelength λb of the laser are transmitted, so that most of the incident light penetrates to the cleaved end face. Further, when the injection current into the second to fourth Bragg reflector regions is significantly increased, the light penetration length can be significantly reduced due to the absorption of light by free electrons, and at least the first The area length of the Bragg reflector area can be reduced.

(3) According to the third aspect of the invention, since the composition of the waveguide forming the Bragg reflector region is set to be equal to or longer than the oscillation wavelength, current injection is performed. By doing so, the medium becomes transparent with respect to the incident light wavelength, and it can be made to be absorbed when current injection is not performed. For example, the medium becomes transparent to the first Bragg reflector region. Current injection is performed and the second
From the first to the fourth Bragg reflector regions, the effective penetration length is about 200 μm, and when uniformly implanted, 600 μm, and the second to the fourth Bragg reflector regions. The refractive index is changed by adjusting the injection amount, and the second wavelength with respect to the oscillation wavelength λb of the laser is changed.
It is possible to increase the penetration depth up to 1600 μm by setting so that the first to fourth Bragg reflector regions are transparent.

(4) According to the invention described in claim 4, the composition of the waveguide forming the Bragg reflector region is a bulk or quantum well structure having a wavelength shorter than the oscillation wavelength by about 50 nm. By applying a reverse bias, the refractive index is changed by the Pockels effect in the case of a bulk and the quantum confinement effect in the case of a quantum well structure, and the second to fourth Bragg reflections with respect to the oscillation wavelength λb of the laser are changed. If you set it so that the mirror area is transparent,
It is possible to increase the penetration depth up to 1600 μm.

[Brief description of drawings]

FIG. 1 is a side view schematically showing a structure of a semiconductor pulse laser device according to an embodiment of the present invention.

FIG. 2 is a diagram showing wavelength characteristics of a reflection coefficient R and a transmission coefficient T of a distributed reflector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 n-side ohmic electrode 2 n-InP clad layer 3,5 InGaAsP guide layer 4 semiconductor multiple quantum well structure 6 p-InP clad layer 7 diffraction grating 8-13 p-side ohmic electrode 21 saturable absorption region 22 gain region 23 Bragg reflection Mirror area 31 Voltage source 32-36 Current source

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hiroshi Ogawa 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd.

Claims (4)

[Claims] 1. A semiconductor pulse laser device in which a saturable absorption region, a gain region, and a Bragg reflector region which are composed of semiconductor waveguides having the same composition are integrated, and the Bragg reflector region is divided and the regions between these regions are divided. A semiconductor pulse laser device characterized in that the penetration length of incident light is changed by changing the refractive index and the absorption coefficient, and the repetition frequency of the mode-locked laser can be changed. 2. The semiconductor pulse laser device according to claim 1, wherein the Bragg reflector area is divided into first to fourth reflector areas, and a current is supplied to all of the first to fourth Bragg reflector areas. No injection was performed, and laser oscillation was generated at the oscillation wavelength determined by the Bragg wavelength λb in these Bragg reflector regions, and then tuning was performed for the second to fourth Bragg reflector regions. The semiconductor pulse laser device is characterized in that the second to fourth Bragg reflector regions for the oscillation wavelength λb of the laser are transmitted by performing the operation. 3. The semiconductor pulse laser device according to claim 1, wherein the waveguide constituting the Bragg reflector region has a composition equal to or longer than the oscillation wavelength. Pulse laser device. 4. The semiconductor pulse laser device according to claim 1, wherein the composition of the waveguide forming the Bragg reflector region is a bulk or quantum well structure having a wavelength shorter than the oscillation wavelength by about 50 nm. Characteristic semiconductor pulse laser device.