JPH0789593B2 - Resonator type cascade optical amplifier - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical amplifier used for amplification of an optical signal and other purposes in optical fiber communication. In particular, the present invention relates to an optical amplifier suitable for being inserted into a portion of an optical fiber transmission line that does not require complete repeat processing and used for the purpose of increasing the repeat interval.

Here, the optical amplifier means an all-optical element that directly amplifies an incident optical signal without performing photoelectric conversion and sends the amplified optical signal.

[Conventional technology]

Conventionally, a semiconductor laser type optical amplifier using stimulated emission of a semiconductor laser has a traveling wave (TW) type that amplifies an optical signal by passing it once and a resonance in which the amplifier itself constitutes a Fabry-Perot resonator. There is a vessel (EP) type.

In the semiconductor laser type optical amplifier, the gain can be increased by the injection current, but in reality, the amplified spontaneous emission light increases as the injection current increases, and the gain saturates even if the injection current increases. Further, as the intensity of the incident optical signal is higher, the saturation of gain is more likely to occur.

In the traveling wave type optical amplifier, this saturation value is an output power of 4 to 8 mW in an aluminum gallium arsenide semiconductor laser (AlGaAs-LD) having a threshold value of 60 to 80 mA.

Since the resonator type optical amplifier is an optical resonator itself, it is possible to greatly increase the gain, but in actual characteristics, the optical output that can be taken out due to the saturation of the gain reaches a ceiling.

Further, since the peak frequency of the gain of the traveling wave type optical amplifier fluctuates due to the temperature change, if the bandwidth is set to 3 to 4 × 10 ³ GHz and the deviation of the peak value of the gain is allowed to be ± 500 GHz, ±
The temperature may be controlled at 5 ° C.

On the other hand, in the resonator type optical amplifier, the bandwidth is extremely narrow because the resonance characteristic is used, and for example, the bandwidth at which a gain of 25 dB can be obtained is 2.5 GHz. This value is much smaller than the amount by which the wavelength of the semiconductor laser fluctuates due to changes in practical temperature and current, and is ± 0 to operate the resonator optical amplifier normally.
Temperature control within 04 ℃ and current control within ± 0.15mA are required. Further, automatic frequency control is indispensable so as to track the frequency of the optical signal transmitted.

When inserting the optical amplifier into the optical fiber transmission system, the polarization state of the signal light propagating through the optical fiber is not determined, so
It is desirable that the optical amplifier has no polarization dependence. However, the active layer of a semiconductor laser is a thin film waveguide, and
The confinement factor Γ in the active layer for modes and TM modes differs depending on this mode.

In the traveling wave type optical amplifier, this difference is the difference in gain with respect to the polarized light. For example, the confinement factor for TE mode Γ _TE =
In a channeled substrate planar semiconductor laser (CSP-LD) with a confinement coefficient Γ _TM = 0.47 for the TM mode of 0.52, the TM mode gain is about 2.4 dB lower when the TE mode gain is 25 dB.

In the resonator type optical amplifier, the situation is worse because the difference of the single pass gain greatly contributes to the overall gain. Furthermore, the reflectance and the resonance frequency of the resonator are different between the TE mode and the TM mode. The polarization dependence of the mold is essentially larger than that of the traveling wave type.

[Problems to be solved by the invention]

In order to increase the gain of such a conventional optical amplifier,
It has been proposed to arrange the semiconductor laser type optical amplifiers in a cascade, but in that case, the following problems occur.

In the traveling wave type optical amplifier, the spontaneous emission light is amplified, so that the gain is saturated, and therefore the output power cannot be increased so much even by simple cascade connection.

Since the resonator type optical amplifier itself is an optical resonator, it is easy to increase the gain, but there is a problem that the bandwidth becomes extremely narrow due to this resonance characteristic. That is, the temperature control needs to be controlled within ± 0.04 ° C., which is a very severe condition even compared with ± 5 ° C. of the traveling wave type optical amplifier.

In addition, since the reflectivity and resonance frequency of the resonator are different between TE mode and TM mode, the polarization dependence is essentially larger than that of the traveling wave type. Therefore, in the case of the resonator type optical amplifier, this polarization dependence is eliminated. Some measure to do so is required.

Further, in the resonator type optical amplifier, an optical isolator is required in order to prevent the reflected light from the incident side reflecting mirror from being injected into the preceding stage optical amplifier in the case of cascade connection.

The problem common to both types of optical amplifiers is that when semiconductor lasers manufactured independently are configured as cascade optical amplifiers, the parameters of the semiconductor lasers differ slightly, so that the gains can be controlled independently by temperature control. The distributions need to match. Especially in the case of a resonator type optical amplifier, temperature control within ± 0.04 ° C is required, but if this is performed for each optical amplifier, the scale of the temperature control circuit becomes too large to be practical. There are drawbacks.

These conventional techniques are described in JC Simon, "Optical Amplification Technology in Optical Communication Systems", Foreign Communication Technology, June 1985.
Mon, pp26-33 (JCSimom, “Light Amplifiers in Opti
cal Communication Systems ", NOTO ASI Series E No.7
See 8) for details.

The present invention solves such conventional problems,
An object of the present invention is to provide a resonator type cascade optical amplifier which has a high gain and whose temperature can be easily controlled.

[Means for solving problems]

A first aspect of the present invention is a resonator-type cascade optical amplifier including a plurality of resonator-type optical amplifiers for amplifying optical signals and an optical system for connecting the plurality of resonator-type optical amplifiers in cascade. The plurality of resonator type optical amplifiers have a structure in which a plurality of resonator type active layer stripes are formed on the same semiconductor substrate, and an optical isolator inserted in the optical system or the input system or the output system of the optical signal. In addition to the configuration of the first invention, the optical system includes an element for connecting even number of resonator type optical amplifiers formed on the same semiconductor substrate by rotating the polarization plane of the optical signal by 90 degrees. It is characterized by

The element that rotates the polarization plane by 90 degrees is preferably a polarization-maintaining optical fiber with a twist of 90 degrees or a TE-TM mode converter.

A second invention of the present invention is, in addition to the configuration of the first invention including a control circuit for performing gain control of a resonator type optical amplifier,
The control circuit is a part of a plurality of resonator type active layer stripes formed on the same semiconductor substrate, and is a means for operating as a photodiode by reverse biasing a stripe adjacent to a stripe used as an optical amplifier. A means for monitoring an output coupled to this stripe as a leaked light from an adjacent stripe, and a means for controlling the gain of the optical amplifier by this monitor output are included.

[Work]

According to the present invention, a gain distribution of a resonator type active layer stripe is crystallized by separately forming active layer stripes having a resonator structure connected in a cascade via an optical isolator on a single semiconductor substrate. Matching can be achieved at the growth stage and the overall temperature can be controlled and tuned to the transmitter semiconductor laser wavelength. This allows
The burden of temperature control is reduced.

In addition, the resonator-type optical amplifier, which has essentially large polarization dependence, is equivalently rotated by 90 degrees around the optical axis and arranged in a cascade to arrange orthogonal polarization components. Amplification by each optical amplifier realizes an optical amplifier having no polarization dependence.

〔Example〕

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

FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, reference numerals 1, 3, 5, 7, and 9 are optical isolators, and reference numerals 2, 4, and 8 are resonator type active layer stripes, which are connected in cascade.

It has the optical amplification function of the active layer. One of the features of the device of the present invention is that a plurality of resonator type active layer stripes are formed on a single semiconductor substrate. When the active layer is of Fabry-Perot (FP) type or distributed feedback (DFB) type, it does not need an optical filter because it has a filter function by itself, but in order to avoid the reverse injection phenomenon due to reflected return light, Use an isolator. When inserting a plurality of cascaded optical amplifiers in the transmission path, either one of the optical isolator 1 and the optical isolator 9 may be used. The number of optical isolators is the maximum [the number of active layers + 1], and the number may be less than that.

FIG. 2 is a block diagram showing an example in which the resonator type cascade optical amplifier of the present invention is implemented in an optical amplification repeater. The optical amplification repeater is configured to directly amplify an optical signal, instead of photoelectrically converting the optical signal and electrically amplifying it. In addition,
In FIG. 2, a plurality of resonator type active layer stripes are formed on the same semiconductor substrate, and some of the plurality of resonator type active layer stripes formed on the same semiconductor substrate are adjacent to each other to operate as a photodiode. Then, the output of this stripe is monitored to control the gain of the amplifier.

In FIG. 2, active layer stripes 201 to 210 are arranged in parallel at intervals of about 10 μm, and an input optical signal is incident on a preamplifier active layer stripe 202 via an optical isolator 301 as necessary. Let The output amplified here is the active layer stripes 204, 2 connected in cascade.
Enter in 06. Optical isolators 302, 304, 306 are arranged at the respective outputs of the active layer stripes 202, 204, 206.
The control circuit 100 receives the output of the active layer stripe 201 as an input and outputs a control signal for controlling the gain to the active layer stripes 204, 2
Output to 06. This constitutes an automatic gain control (AGC) optical amplifier. The output of the active layer stripe 206 at the final stage of the AGC optical amplifier is input to the main amplifier active layer stripes 208, 209, and 210 via the optical isolator 306, and the output of each active layer stripe is combined by the optical coupler 400 to generate an optical signal. It is output to the fiber transmission line.

The output of the preamplifier active layer stripe 202 is cascaded to form an AGC optical amplifier active layer stripe 20.
It is amplified to a predetermined level at 4,206. The gain control of this AGC optical amplifier is performed as follows. Active layer stripe 201 located outside of preamplifier active layer stripe 202
Is reversely biased to operate as a photodiode. Part of the light in the preamplifier active layer leaks into the adjacent photodiode stripe (201) and is optically coupled. This allows the preamplifier active layer stripe 202
Since a photocurrent proportional to the emission intensity of is obtained from the photodiode stripe (201), the gain of the AGC optical amplifier is controlled by processing this with the control circuit 100.

Here, each stripe portion of the active layer stripes 203, 205 and 207 may be used as a photodiode similarly to the active layer stripe 201, but essentially, the active layer stripes (202, 204) for amplification of each of them. , 206) to act as an absorption layer for optically separating them.

By configuring the main amplifier with the plurality of active layer stripes 208, 209, 210, it is possible to mitigate the effect of gain saturation caused by a large input optical signal and to increase the output power of the transmitted optical power. Further, in the case of this embodiment in which light is coupled by leakage light, three active layer stripes 208, 209 and 21 are formed.
Since 0 oscillates in a phase-locked manner, optical signals whose phases are aligned are sent to the optical coupler 400.

With such a configuration, an optical signal repeater that directly amplifies an optical signal can be realized.

FIG. 3 is a block diagram showing the first embodiment of the present invention. The first feature of the present invention is that, when a resonator type optical amplifier is used, the reflectance and the resonance frequency of the resonator are different in both TE and TM modes, so that the polarization dependence is a traveling wave type optical amplifier. However, the paired resonator-type optical amplifiers are equivalently rotated 90 degrees around the optical axis and arranged in a cascade, and orthogonal polarization components are arranged in respective optical amplifiers. By amplifying with, the optical amplifier having no polarization dependence is being realized. The optical isolator is omitted in FIG.

FIG. 3 (a) shows a polarization-preserving optical fiber 501 twisted by 90 degrees is used in an optical system for coupling resonator-type active layer stripes 221 and 222 formed on the same semiconductor substrate in a cascade.
Adjacent resonator type active layer stripes 221 and 222 are equivalently rotated 90 degrees around the optical axis and arranged in a cascade. With such a configuration, the output polarization state of the resonator-type active layer stripe 221 can be rotated by 90 degrees and incident on the next resonator-type active layer stripe 222, and the TE mode and the TM mode can be switched. Therefore, as a result, an optical amplifier having no polarization dependence can be constructed. In addition,
For that purpose, it is preferable that the number of active layer stripes used for optical amplification is even.

Further, in general, a polarization-maintaining optical fiber holds the polarization plane by suppressing mode conversion by increasing the propagation constant difference between the orthogonal linear polarization modes. For this reason, a group delay difference of each polarization mode usually occurs. When the pulse transmission of light becomes faster, it is expected that this difference in group delay causes the spread of pulses. In this case, the polarization-maintaining optical fiber 501 is used so as to cancel this group delay difference.
A polarization-maintaining optical fiber 502 having the same characteristics as and having the same length as that of the resonator type active layer stripe may be connected to the input side or the output side of the resonator type active layer stripe (in this embodiment, the resonator type active layer stripe 22
2 is connected to the output side).

Fig. 3 (b) shows TE as an element that rotates the plane of polarization by 90 degrees.
An embodiment using the TM mode converter 601 will be shown. Since the TE-TM mode converters 601 and 602 are used instead of the polarization-maintaining optical fiber twisted by 90 degrees, the same effect can be obtained. TE-TM
For the mode converter, refer to: RC Alphaness, L.
L. Boulle, "Low Drive Voltage TE-TM Mode Converter", Optics Letter, Volume 5, Issue 11, 473, 1980 (RCAlf
aness and LLBuhl, “Electro−optic TE−TM mode co
nverter with low drive voltage ", OPTICS LETTERS, Vo
l.5, No. 11, p. 473, 1980).

In the optical transmission system using the single mode optical fiber,
This is often used in the wavelength band where the effect of dispersion is negligible, and since the repeater interval is limited only by the optical loss of the system at this time, instead of a complex optical repeater that performs complete identification regenerative repeater, An optical amplifier that directly amplifies light can be applied. In such a case, if the cascade optical amplifier of the present invention is used, only one temperature control circuit is required for each semiconductor laser in the conventional optical amplification technique, and the feasibility can be greatly improved. it can.

〔The invention's effect〕

As described above, the present invention forms a series of cascaded active layer stripes on a single semiconductor substrate, so that the characteristics of each stripe are uniform. Therefore, the deviation of the maximum gain frequency between the stripes becomes small, and the effect of the cascade connection becomes large. Also, it is not necessary to provide an independent temperature control circuit for each amplifier, and only one circuit for controlling the entire temperature is required. Especially, in the case of a resonator type optical amplifier, the temperature is within ± 0.04 ° C. Since the temperature control is required, the temperature control becomes easy and the manufacturability can be greatly improved.

In addition, a resonator-type optical amplifier with a large polarization dependence is equivalently rotated 90 degrees around the optical axis and arranged in a cascade, and the orthogonal polarization components are amplified by the respective optical amplifiers. Can realize an optical amplifier,
Therefore, it can also be used as an optical amplifier in an optical fiber transmission line whose polarization state is not fixed.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention. FIG. 2 is a block diagram showing an embodiment of the present invention applied to an optical amplifier repeater. FIG. 3 is a block diagram showing the configuration of an optical amplifier having no polarization dependence. 1, 3, 5, 7, 9 ... Optical isolator, 2, 4, 8 ...
… Resonator type active layer stripe, 100 …… Control circuit, 201〜
210, 221-224 ... Active layer stripes, 301, 302, 304,
306 ... Optical isolator, 400 ... Optical coupler, 501,502 ...
… Polarization maintaining fiber, 601, 602… TE-TM mode converter.

Claims (4)

[Claims] 1. A resonator-type cascade optical amplifier comprising a plurality of resonator-type optical amplifiers for amplifying optical signals, respectively, and an optical system for connecting the plurality of resonator-type optical amplifiers in a cascade. The resonator type optical amplifier is a structure in which a plurality of resonator type active layer stripes are formed on the same semiconductor substrate, and includes an optical isolator inserted in the optical system or an input system or an output system of an optical signal, The optical system includes a resonator cascade optical amplifier including an element that connects a plurality of resonator optical amplifiers formed on the same semiconductor substrate by rotating the polarization plane of an optical signal by 90 degrees. 2. A resonator-type cascade optical amplifier according to claim 1, wherein the element for rotating the plane of polarization by 90 degrees is a polarization-maintaining optical fiber having a twist of 90 degrees. 3. A resonator-type cascade optical amplifier according to claim 1, wherein the element that rotates the plane of polarization by 90 degrees is a TE-TM mode converter. 4. A plurality of resonator type optical amplifiers for respectively amplifying optical signals, an optical system for connecting the plurality of resonator type optical amplifiers in a cascade, and a control circuit for performing gain control of the resonator type optical amplifiers. In the resonator-type cascade optical amplifier, the plurality of resonator-type optical amplifiers have a structure in which a plurality of resonator-type active layer stripes are formed on the same semiconductor substrate. The control circuit includes an optical isolator inserted in an input system or an output system, and the control circuit is a part of a plurality of resonator type active layer stripes formed on the same semiconductor substrate and is used as an optical amplifier. Means to reverse-bias the stripe adjacent to and to operate as a photodiode, and monitor the output coupled to this stripe as leakage light from the adjacent stripe. It means a resonator cascade optical amplifier characterized in that it comprises a means for controlling the gain of the optical amplifier by the monitor output.