JPH01102983A - Light amplifier - Google Patents

Abstract

PURPOSE:To monolithically compose a light amplifier of a semiconductor material, and obtain the amplifier having no polarizing dependency characteristic to incident light without necessity of a complicated control system by varying the polarizing state of an input light to a semiconductor laser light amplifier utilizing the gain mechanism of a semiconductor laser at a speed sufficiently higher than that of a signal to be amplified. CONSTITUTION:A signal light 2 transmitted from a signal mode optical fiber(SMF)1a is coupled by coupling means to the waveguide 5 of a polarizing rotator 3. The rotator 3 rotates its polarizing direction in response to a drive signal. The output light of the rotator 3 is coupled to the waveguide 3 of an LD light amplifier 4 to DC-drive the amplifier 4, thereby amplifying the input signal light 2. The amplified light 2 is again coupled by the coupling means to an SMF1b to be transmitted. Here, if the rotator 3 is driven at a speed sufficiently higher (here 1GHz) than the speed of the transmitted signal (here 140Mb/S), the polarizing state of the input light to the amplifier 4 has in average halves of TE and TM components. Accordingly, a stable light amplification is performed without variation in the intensity of the output light due to a disturbance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical amplification device, and more particularly to an optical amplification device using a semiconductor laser type optical amplifier used in fields such as optical communication and optical switching.

[Traditional skill paddle]

Conventionally, optical amplifiers have been used to extend optical communications over long distances and increase capacity.

It is an essential device for purposes such as increasing the scale of optical switching systems. Although it is possible to use such an optical amplifier that utilizes nonlinear scattering within an optical fiber, a semiconductor laser (LD) type is used because of its advantages such as small size, high efficiency, and possibility of integration with other semiconductor optical devices. ing. This LD
The optical amplifier has an internal gain of 20 to 39 dB, and a gain of about 20 dB between seven optical fibers when optical fibers are connected to the input and output ends. In addition, due to recent advances in anti-reflection (AR) coating technology on end faces, saturated light output has been improved.

The gain wavelength band has also been significantly expanded, making the device close to practical.

[Problem that the invention seeks to solve]

However, conventional LD optical amplifiers have a problem in that their characteristics largely depend on the polarization state of incident light. In other words, in a long-distance single mode optical fiber (8MF) under normal usage conditions, the polarization state of the incident light is not preserved;
Furthermore, the polarization state of the propagating light changes greatly depending on external temperature, pressure, etc.

Therefore, when inserting an LD optical amplifier in the middle of an SMF, the output light intensity will fluctuate greatly unless some kind of bias control means is used.

There are three primary reasons why the characteristics of such an LD optical amplifier have dependence on incident polarization. −゛(1) Polarization dependence of gain itself (21 Difference in confinement coefficient in active layer depending on polarization (3)
Polarization value of end face ripple rate! In a normal double heterostructure LD optical amplifier, (1)
The gain itself does not exhibit polarization dependence.

Further, in principle, the problems of +21 and 131 can be solved by making the waveguide structure of the active layer isotropic and reducing the end face reflectance. However, such problems were discussed, for example, at the First Optoelectronics Conference held in Tokyo in July 1986.
``Po5t Deadline Papers Technical Digest J (Po5t Deadline Papers T
mechanical Digest) B 11-2
, pages 12-13, and according to this published paper, an LD with a buried heterostructure with an isotropic waveguide structure.
Even in a traveling wave type LD optical amplifier, which has an extremely high quality AR coating with a reflectance hole of 0.04% on both end faces,
A maximum gain difference of 10 dB or more has been observed between horizontally polarized (TE) and vertically polarized (TM) light on both sides.

FIG. 3 shows the TE of the above-mentioned normal traveling wave LD optical amplifier.
FIG. 3 is a gain characteristic diagram for both TM and TM lights.

As shown in FIG. 3, the signal gain of the conventional LD optical amplifier is likely to be higher in TE than in TM by more than 1 QdB at most.

In short, it is difficult to reduce the polarization dependence of the characteristics of an LD optical amplifier only by making the waveguide structure isotropic and reducing the end face reflectance.

Two ways to solve this problem are to use polarization controllers in combination. However, since it is difficult to realize a small-sized, low-voltage (low-current) polarization controller using semiconductor materials, monolithic integration is difficult, and therefore a complicated optimal control system must be used.

An object of the present invention is to eliminate such problems and provide an optical amplification device which can be monolithically constructed using semiconductor materials, does not require a complicated control system, and whose characteristics do not depend on incident polarization.

[Means for solving problems]

The optical amplification device of the present invention includes a semiconductor laser optical amplifier that utilizes the gain mechanism of a semiconductor laser, and a polarization state of input light to the semiconductor laser optical amplifier at a speed sufficiently higher than the speed of a signal to be amplified. and means for changing.

[Effect]

As shown in Figure 3, the T of a normal traveling wave LD optical amplifier is
The signal gains for both the E and TM lights are generally higher in the TE mode than in the TM mode because the light confinement coefficient within the active layer is higher. Therefore, the dependence of the gain of the LD optical amplifier on the incident polarization becomes a problem because, as mentioned earlier, the polarization state of the light propagating in the normal SMF fluctuates over time depending on the external conditions. There is a possibility that this is due to fluctuations in the intensity of the output light. As can be seen from FIG. 3, a considerably large gain can be obtained even for TM polarized light. Therefore, even if the polarization state of the input light does not necessarily match the polarization state that gives the maximum gain (TE in this case), as long as it does not fluctuate over an hour, the output light intensity will not fluctuate, and this is not a big problem in practice. Does not occur.

The optical amplification device of the present invention takes advantage of this fact and fixes the polarization state of the input light to the LD optical amplifier to be half TE and half TM. This TE and TM acid components are each 1/2
The gradual polarization state can be obtained by forcibly changing the polarization state of the input signal of the LD optical amplifier at a speed sufficiently faster than the signal speed of the input signal. Moreover, the optical amplification device of the present invention does not require a complicated feedback control system that is required in a normal polarization controller. That is, since such a means for changing the polarization state can be easily made using a semiconductor material, it becomes possible to form a chinolithic structure with an LD optical amplifier. Therefore, the polarization state of the input light is TE, TM
By reducing each of the acid components to 1/2, the gain obtained in the 9.LD optical amplifier becomes the average value of the gain for the both-side optical state, but a sufficiently high value can still be obtained.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a front view of an optical amplification device for explaining one embodiment of the present invention.

As shown in Figure 1, single mode optical fiber≠(aMF
) The signal light 2 transmitted from la is coupled to a coupling means (not shown)
It is then coupled to the waveguide 5 of the polarization rotator 3. This polarization rotator 3 rotates the polarization direction in accordance with a drive signal, and details of this part will be explained later. Polarization rotator 3
The output light is coupled to the waveguide 5 of the LD optical amplifier 4, and by driving the LD optical amplifier 4 with DC, the input signal light 2
is amplified. This amplified signal light 2 is again coupled to 8 MF 1 by the coupling means and transmitted. Here, if the polarization rotator 3 is driven at a sufficiently high speed (l GHz here) relative to the signal speed being transmitted (here 140 Mb/8), the polarization state of the input light to the LD optical amplifier 4 will be averaged. Generally speaking, it has half TE and half TM acid components. This state is not affected even if the transmission state within aMF l a changes due to disturbances. Therefore, stable optical amplification is possible without fluctuations in the output light intensity due to disturbances.

Next, the polarization rotator 3 will be described in detail.

FIG. 2 is a sectional view of the polarization rotator shown in FIG. 1. still,
Here, we will mainly explain the principle, so InGaAsP
This will be explained using a planar structure, which is the simplest type of semiconductor.

As shown in FIG. 2, such a polarization rotator 3 has n+-(t
lo) n”-InP crat layer 22 on the InP substrate 21
, i-InGaAsP waveguide layer 23. P-In
P class, de layer 24. p-InGaAsP cap layer 2
It is manufactured using a wafer on which 5 is grown. Next, n
Ohmic electrodes 26a and 26b are formed at the cleavage and pH points, respectively, and then AR coating films 278 and 27b for antireflection are formed on the cleaved end faces.

This structure is a p-i-n diode structure.
By applying a reverse bias voltage between the electrodes 26a and 26b, an electric field is applied to the waveguide layer 23, causing a stain optical effect.

Among these electro-optic effects, the electro-optic effect due to the application of an electric field in the direction perpendicular to the (110)≠ plane is described, for example, in the magazine "Journal of Applied Fixtures/C (
Journal of Ap-≠plied Phys
ics ) J, Volume 47 (published in 1976) 206
This is discussed in detail in the paper published on pages 9-2078.

Also, due to the electric field in the (110) direction, 9InP.

In a crystal belonging to the crystal point group (43m) K, such as GaAs, polarization rotation occurs instead of phase modulation. This effect is known to be the most efficient of the electro-optical effects of (43m) crystals, and a value of TE-+TM conversion voltage of less than 5V has been reported for a 2.2mm long GaAs-based element. .

Moreover, since this structure is used in a reverse bias state, the element response speed is not limited by the carrier lifetime, but is determined by the CR time constant determined by the element capacitance C and series resistance RK. Therefore, it is possible to easily obtain a response band of GHz or more depending on the element structure, and therefore it is possible to rotate the polarization of the input light to the LD optical amplifier at a sufficiently high speed compared to the signal speed to be transmitted. can.

Although the above element structure has been described here as a planar structure without a horizontal transverse mode control structure for simplicity, it is of course possible to introduce other transverse mode control structures such as a lip-embedded type.

Next, the LD optical amplifier will be explained. L used here
D optical amplifier 4 in Figure 1 is a Fabry-Perot type DC-P
AR coating (RC
1%).

This structure is the magazine "Electronics Letters J (Electronics Letters) Volume 21.

(1985), pp. 501-502. Namely.

~3Qd for TE polarized input at current near threshold
A difference of 10 dB between the B gain, TE, and T gain was obtained.

Using the components of the polarization rotator 3 and the LD optical amplifier 4 as described above, the configuration shown in FIG.
When an amplification test was performed on the signal, an inter-fiber gain of ~15 dB was obtained, and no gain fluctuations due to disturbance were observed.

In addition, in the embodiment described above, the polarization rotator 3 is (110
) substrate, and since the LD optical amplifier 4 is formed on a (100) substrate, simple monolithic integration is difficult as it is.

However, since the LD optical amplifier can also be manufactured on a (110) substrate, it is also possible to obtain an optical amplification device in which the polarization rotator 3 and the LD optical amplifier 4 are monolithically integrated by using a (110) substrate. It is possible.

〔Effect of the invention〕

As described above, the optical amplification device of the present invention can be monolithically constructed using semiconductor materials, and has the advantage that it can be obtained without requiring a complicated control system and with characteristics free from dependence on incident polarization.

[Brief explanation of the drawing]

FIG. 1 is a front view of an optical amplification device for explaining an embodiment of the present invention, FIG. 2 is a cross-sectional view of the polarization rotator in FIG. 1, and FIG. 3 is a polarization dependence of gain in a conventional optical amplifier. FIG. 3 is a current/gain characteristic diagram for explaining the characteristics. la, lb...single mode optical fiber, 2...
...Signal light, 3...Polarization rotator, 4...
...LD optical amplifier, 5... Waveguide, 21...
...n" (1110) InP substrate, 22...
...n''-InP cladding layer, 23...1-
InGaAsP waveguide layer, 24...p-I n
P cladding layer, 25-.-p-InGaAsP capping layer, 26a. 26b...Ohmic electrode, 27a, 27b...
...AR coat film. Agent Patent Attorney Susumu Uchihara

Claims (1)

[Claims] The present invention is characterized by comprising a semiconductor laser optical amplifier that utilizes a gain mechanism of a semiconductor laser, and means for changing the polarization state of input light to the semiconductor laser optical amplifier at a speed sufficiently higher than the speed of the signal to be amplified. optical amplification device.