JPS6289378A - Semiconductor laser device with stabilized frequency - Google Patents

Abstract

PURPOSE:To contrive the shortening of a spectrum width and the stabilization of absolute frequency by using a semiconductor laser comprising an external resonator and an optical waveguide monolithically into one body and converting a frequency fluctuation of the laser beam into an intensity fluctuation by use of etalon, and accepting it by a detector and feeding back the accepted light signal to a semiconductor laser drive power source and a temperature controller electrically. CONSTITUTION:An emitted laser beam 20 of a semiconductor laser 18 comprising an optical waveguide 1 arranged along the optical axis direction of an active region 10 and being transparent to the oscillation wavelength of said region, and being provided with a laser resonator composed of cleavage planes 14 and 16 is nearly collimated by a lens 22. That beam passes through an isolator 24 to remove an influence of reflected light on the semiconductor laser and passes an etalon 26 comprising a proper free spectrum region (FSR). Then it is accepted by a detector 28. The accepted signal is fed back to a semiconductor laser drive power source 32 and a temperature controller 38 by feedback loops 30 and 40 so as to restrain a variation in intensity.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a technique for narrowing the oscillation spectrum line width and stabilizing the absolute frequency of a semiconductor laser in optical fiber communications, optical fiber sensors, and the like.

Conventional technology The performance of semiconductor lasers has made remarkable progress in recent years.
For example, regarding the unity of the longitudinal mode, sub 2.

A mode whose intensity is less than /10Q) compared to the main mode has been achieved, and the temporal coherence characteristics of the laser have also been improved. Conventional optical communication has mainly been based on intensity modulation by modulating the laser drive current, but recently, improvements in its temporal coherence characteristics have enabled optical communication to have an oscillation frequency of ~10Hz, which is its original feature. One possibility is to use frequency as information. When trying to utilize optical frequencies, the problem is how much the spectral linewidth can be suppressed. For example, when considering coherent communication, the spectral linewidth is required to be 1 MHz or less, and in the case of an original fiber gyro, it must be further reduced. From a practical point of view, it is preferable to use In in the long wavelength band, that is, in the 1.3 μm to 1.6 μm band, where fiber loss is smaller than the AIGILAs semiconductor system in the 0.8 μm band.
It must be realized using P-based materials. In addition to having a narrow spectral linewidth, it is also important that the absolute frequency of the oscillation source must be stable. For example, in coherent communication, the oscillation frequency of locally oscillated light must be stabilized as much as possible, although it depends on the modulation method and the like.

There is a method of narrowing the oscillation spectrum linewidth and stabilizing the absolute frequency as described above, and a method of arranging a mirror outside the semiconductor laser to form a so-called external resonator. For example, it is described in the following documents (1) to (5).

Literature (1): T-Fushi outside 1/'orsion frequency shift thepresion op semiconductor laser cabled toe internal cavity"Electronics Letters, vol. 20, 4
161T (1984) (T, Fujita
at al, ”0scillation
frequency 5hift 5upp
reaction of semiiicomd-u
ctor 1azers coupled to ex
internal cavity”Electron, Let
t,,You,20. P, 416 (1984)') Literature (2): T. Fujita et al. ``Intensity-Noise Suppression and Moderation Characteristics of Laser Diode Force Pulled Toe Internal Cavity'', IEE Journal of Quantum Elec]
・Lonics Ql-2C4, 4921ff (1984) J':T, Fujita et, al
,, “Intensity Noise 5up
pressure ancl modulation
naharactics of 1az
er diocie coup -1ed to
external cavity,”IEKE
J.

Quantum Electron, Vol, QR
-20, P, 492 (1984)] Literature (3): T. Fujita et al. °' Correlation Between Inlancity Noise and Longitudinal Distance Opa Semiconductor Laser Force Pull-to-To An Internal Cavity'' Journal Op Applied Physics, Volume 67, 1753i (1985) l”T
-Fujita et, aL, “Go
rre-tion between 1ntens
ity noise and longitudin
al modes of a samicond
uc-tor 1azer coupled t
o an external cavity,”
J, Appl, phys, You, 57. P, 1753
(1986)] Literature (4): H. Zato et al.
ANALYSIS CARBLONZITI CORDINAL MODE CARBLINKIN INTERNAL CAVITY SEMICONDUCTOR RACERS'.

IEEEE Journal of Phantom Electronics, Q]iney vol. 21, 284i'7 (
1985) (H, 5ato at, al,, ”
TheoraTical analysis-sis o
f longitudinal mode co
uplining external cavit
y so m1 conductor lazers,”
IEEE J, Quantum Electron
,.

Vol, QE-21, P, 284 (1985)] Literature (5):
Fractuation of Semiconductor Lasers Coupled Tow Internal Cavity Paiiii Journal Op Phantom Electronics, QE-21, pp. 8, 46 (1985)
[H, 5ato at, al,.

'Tense flux
of semicon-αactor 1azer
s coupled to externalC
aVity, “IREE J, Quantum E
lectron.

Vol. QE-21, P, 46 (1986)') The present inventors also conducted a detailed study on the configuration in which the external vC of the laser is arranged, but the mechanical stability was not determined. From a practical point of view, it was quite difficult to maintain the device stable for several hours. This is due to instability such as vibrations and temperature fluctuations in the experimental system. In other words, placing optical components outside 1-1
Using optical feedback from this method has practical difficulties in terms of long-term stability.

On the other hand, in order to stabilize a certain system as a technology in control engineering, there is a method of configuring an electrical feedback loop and performing so-called negative feedback control. If we consider stabilizing the absolute frequency of, for example, a semiconductor laser using such a method, since the absolute frequency fluctuations of the semiconductor laser are due to temperature fluctuations or injection current fluctuations, it is only necessary to appropriately control them. However, in this case, a semiconductor laser with an original narrow spectral linewidth is required, and only by stabilizing the absolute frequency of the laser with a narrow spectral linewidth can it be used for applications such as coherent communication. Conventionally, there has been no such mechanical technology for stably narrowing the oscillation spectrum line width and stabilizing the absolute frequency over a long period of time.

Problems to be Solved by the Invention 7. Problems to be solved by the present invention are how to stably narrow the spectral linewidth and stabilize the absolute frequency of a semiconductor laser over a long period of time.

In order to solve the problems described in 1. ``Means for Solving Interference Points'', the present invention provides a monolithic external cavity semiconductor laser having an optical waveguide transparent to the oscillation wavelength as an external cavity, a lens, an inlator, and an etalon. and the detector is arranged in the optical axis direction l~, and the light reception signal of the semiconductor laser beam according to the detection of the previous d1 is [)11
The present invention is a frequency-stabilized semiconductor laser device having a driving power source for the semiconductor laser and a means for supplying air feedback to the temperature controller of the semiconductor laser.

The present invention does not use an unstable system such as placing a mirror outside the semiconductor laser, but uses a semiconductor laser having a monolithic integrated structure with an optical waveguide as an external resonator, and the frequency of the laser light is The fluctuation is converted into intensity fluctuation using an etalon, which is received by a detector, and the received light signal is electrically fed back to the semiconductor laser drive power source and temperature controller.

Example Embodiments of the present invention will be described below with reference to the drawings.

As shown in the figure, a semiconductor laser 18 has an optical waveguide 12 that is transparent to the oscillation wavelength in the optical axis direction of the active region 10, and has a laser resonator formed by cleavage planes 14 and 16.
The emitted laser beam 2o is substantially collimated by the lens 22. The light passes through an isolator 24 to eliminate the influence of reflected light on the semiconductor laser, passes through an etalon 26 having an appropriate free spectral range (F'SR) L-1, and is received by a detector 28. The received signal is fed back to the semiconductor laser drive power supply 32 by the feedback loop 30, thereby controlling the amount of current injected into the injection line 36 from the current injection electrode 34 to the semiconductor laser 18. The signal received by the temperature detector 28 is also fed back as a control signal to the temperature controller 38 of the semiconductor laser 18 via the feedback loop 40. When such a system is configured, in the etalon section 269 /.-1, the frequency of the semiconductor laser 18 immediately before the incidence on the etalon section 26 can be converted into an intensity fluctuation immediately after passing through the etalon section 26. can. That is, in the detector 28, a value set to a certain constant,
Intensity fluctuations are suppressed by differentially detecting the fluctuation value as an error signal and electrically feeding it back to the drive power source 32 and temperature controller 38 via feedback loops 30 and 4o. In other words, it is possible to suppress the frequency fluctuation of the semiconductor laser 18 by this means.
4, absolute frequency stabilization becomes possible.

The semiconductor laser used in this embodiment will be described below. The semiconductor laser 18 has an optical waveguide 12 which is monolithically integrated outside the active region 10 and is transparent to the oscillation wavelength. The resonator plane is constituted by cleavage planes 14 and 16. With such a structure, the spectral linewidth of the semiconductor laser can be made extremely narrow, and moreover, it oscillates in a single longitudinal mode. The device actually manufactured by the present inventors as a prototype operated stably with a threshold current of about 60 mA. Only 1
0'・-7 Also, the intensity of the sub-longitudinal mode was less than 1 AOo○, and the spectrum line width was very narrow. Mechanical instability caused by the so-called external cavity configuration using mirrors or gratings can be completely eliminated, and the spectral linewidth can be reduced to about 1/100 or less compared to conventional semiconductor lasers. is made of. That is, a spectral linewidth of 100 KH2 is achieved in the semiconductor laser 18 alone, and such characteristics cannot be achieved with conventional single lasers.As shown in the figure, the optical waveguide 12 is optically coupled to the active region 10, and resonance is achieved. This became possible only by forming the vessel surface with cleavage surfaces 14 and 16.

Furthermore, by detecting frequency fluctuations using the etalon 26 while maintaining such a narrow spectral linewidth and providing electrical negative feedback, the absolute frequency stability can be increased to 100KH2.
The following will be achieved: Furthermore, compared to conventional systems, this system can operate stably for a long period of time without changing at all. Until now, there has been no system that provides such excellent characteristics as 11.\-/7.

Since the isolator 24 is added, for example, the semiconductor laser 18 is not affected by reflected light from the surface of the etalon 26 or the surface of the detector 28 or the like. However, it can be operated stably for a long period of time. The lens 22 used here is desirably coated with an AR coating for the oscillation wavelength of the semiconductor laser 18. For example, resonator surface 14
The emitted laser light is coupled to the optical fiber 42, and the propagated light can be used as a light source with extremely stable absolute frequency and narrow spectral linewidth, for example, in coherent optical communications, optical fiber gyroscopes, and sensors. It becomes possible. Of course, the oscillation wavelength of the laser 18 is not limited to a specific one; that is, a semiconductor laser using an rt-vt group semiconductor such as Zn5e mixed with an AlGaAs semiconductor or an InP semiconductor may be used. The semiconductor laser section may include a diffraction grating for wavelength selection.

Further, the etalon used may be an air gap etalon or a solid etalon, and the A quality is not limited. This may be a semiconductor etalon. Further, various forms other than the embodiments of the present invention may be taken, but those that are conceptually similar to the present invention are included in the present invention.

According to the present invention, as described in 1. Effects of the Invention, it is possible to provide a semiconductor laser which has an extremely stable narrow spectral linewidth and a stable absolute frequency without having the various mechanical instability of the conventional laser. became possible for the first time, and its effects are extremely impressive.

[Brief explanation of drawings]

The figure is a configuration diagram of a frequency-stabilized semiconductor laser device according to an embodiment of the present invention. 10... Active region, 12... Optical waveguide, 14°16... Cleavage plane, 18...
Semiconductor laser, 20... Laser light, 22...
...Lens, 24...Isolator, 26...
...Etalon, 28...Detector, 30.4
0... Feedback loop, 32... Power supply, 3
4... Electrode, 36... Injection line, 38...
...Temperature controller, 42...Optical fiber.

Claims (1)

[Claims] A monolithic external cavity semiconductor laser having an optical waveguide as an external cavity that is transparent to the oscillation wavelength, a lens,
means for arranging an isolator, an etalon, and a detector in order in the optical axis direction, and for electrically feeding back a light reception signal from the semiconductor laser by the detector to a drive power source for the semiconductor laser and a temperature controller for the semiconductor laser; A frequency stabilized semiconductor laser device comprising: