JPH071811B2 - Traveling wave 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 (FP) 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 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 type 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 the channeled substrate planar semiconductor laser (CSP-LD) with a confinement coefficient Γ _TM = 0.47 for the TM mode, 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,
It is necessary to control the temperature within ± 0.04 ° C, which is a very severe condition compared to ± 5 ° C for traveling wave optical amplifiers.

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. In particular, 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 is too large to be practical. There is a drawback.

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 Optic
al Communication Sistems ", NOTO ASI Series E No.7
See 8) for details.

The present invention solves such conventional problems,
It is an object of the present invention to provide a traveling wave 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 traveling wave cascade optical amplifier including a plurality of traveling wave optical amplifiers for amplifying optical signals, respectively, and an optical system for connecting the plurality of traveling wave optical amplifiers in a cascade. The plurality of traveling wave type optical amplifiers have a structure in which a plurality of traveling wave type active layer stripes are formed on the same semiconductor substrate, and an optical filter inserted in the optical system or an input system or an output system of an optical signal. It is characterized by including.

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 traveling wave optical amplifier,
The control circuit is a part of a plurality of traveling wave type active layer stripes formed on the same semiconductor substrate, and is configured to operate as a photodiode by reverse biasing a stripe adjacent to a stripe used as an optical amplifier. And a means for monitoring the 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.

The optical filter is preferably a diffraction grating formed on the same semiconductor substrate.

It is preferable that the optical system is formed on the same semiconductor substrate.

[Action]

According to the present invention, traveling wave type active layer stripes formed separately on a single semiconductor substrate are connected to a cascade via an optical filter. The gain saturation due to the amplification of is suppressed.

In addition, even if the cascade connection is used, the gentleness of the temperature condition peculiar to the traveling wave type is not impaired, and by forming the active layer stripes on a single semiconductor substrate, it is possible to make the gain distributions consistent with each other. The temperature can be controlled and tuned to the wavelength of the semiconductor laser on the transmitting side, and the burden of temperature control is reduced.

〔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 filters, and reference numerals 2, 4, and 8 are traveling wave active layer stripes, which are connected in cascade.

The active layer has a light amplification function. One of the features of the device of the present invention is that a plurality of traveling wave active layer stripes are formed on a single semiconductor substrate. Since the active layer is a traveling wave type optical amplifier that has neither a reflection element on its input nor output, an optical isolator is not required, but the light to be amplified is cut out in the frequency domain to avoid gain saturation as much as possible. An optical filter is used to do this. When inserting a plurality of cascaded optical amplifiers in the transmission line, either one of the optical filter 1 and the optical filter 9 may be used. The number of optical filters 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 traveling wave 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,
FIG. 2 shows a first invention of the present invention in which a plurality of traveling-wave active layer stripes are formed on the same semiconductor substrate, and a plurality of traveling-wave active layer stripes formed on the same semiconductor substrate. An example of the second invention of the present invention is shown in which a part of the adjacent stripes of the above is operated as a photodiode, and the output of the stripe is monitored to control the gain of the amplifier.

In FIG. 2, the active layer stripes 201 to 210 are arranged in parallel at intervals of about 10 μm, and the input optical signal is incident on the preamplifier active layer stripe 202 via the optical filter 301 as necessary. Let The output amplified here is output to the active layer stripes 204 and 206 connected in cascade.
To enter. Optical filters 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 and 206. 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 filter 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, 207 may be used as a photodiode similarly to the active layer stripe 201, but essentially, the active layer stripes (202, 204, 206) acts as an absorption layer for separating light optically.

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.

Usually, the active layer of a semiconductor laser is a thin film waveguide, TE,
The TM mode gain is generally lower than the TE mode gain because the confinement factor Γ in the active layer for both TM modes is different. Therefore, if this causes a problem, the cross section of the waveguide may be made square to degenerate the orthogonal polarization mode.

In the case where optical filters and optical components for cascade connection are individually prepared and are optically connected through an air layer, reflection at each end face becomes a problem. Therefore, it is desirable to make those optical components in the same semiconductor substrate.

FIG. 3 is a diagram showing a configuration in the case where the optical filter is formed on the same semiconductor substrate as the active layer stripe. As shown in FIG. 3, it can be realized by forming the optical filters 311 and 312 as diffraction gratings inside the crystal. This can be easily manufactured by using the diffraction grating manufacturing technique of the distributed feedback semiconductor laser which is currently put into practical use.

When the active layer stripes 221 and 222 are formed in parallel, as shown in FIG. 3A, the mirrors 501 and 502 formed by etching or the like are used to guide light to the next active layer stripe. Configure In order to omit this mirror, as shown in FIG. 3B, the active layer stripes 223 and 224 may be formed to be inclined.

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.

In addition, by making the cross section of the waveguide square and degenerating the orthogonal polarization mode, it is possible to realize an optical amplifier that does not have polarization dependence, and therefore is applied to optical amplification in an optical fiber transmission line where the polarization state is not fixed. be able to.

〔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. Further, 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, which facilitates temperature control and greatly improves manufacturability. it can.

Also, since the amplification can be performed in the traveling wave type, the temperature condition peculiar to the traveling wave type can be maintained, and therefore the gain bandwidth product (GB product) can be made larger than that of the resonator type. There is an effect that it can be applied to the pulse transmission.

[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 diagram showing a configuration example in the case where an optical filter is formed on the same semiconductor substrate as an active layer stripe. 1, 3, 5, 7, 9 ... Optical filter, 2, 4, 8 ... Traveling wave type active layer stripe, 100 ... Control circuit, 201-21
0, 221-224 ... Active layer stripes, 301, 302, 304, 30
6, 311, 312 …… Optical filter, 400 …… Optical coupler, 501, 5
02 …… Mirror.

Claims (4)

[Claims] 1. A traveling-wave cascade optical amplifier comprising a plurality of traveling-wave optical amplifiers for amplifying optical signals, respectively, and an optical system connecting the plurality of traveling-wave optical amplifiers in a cascade. The traveling wave type optical amplifier has a structure in which a plurality of traveling wave type active layer stripes are formed on the same semiconductor substrate, and includes an optical filter inserted in the above-mentioned optical system or optical signal input system or output system. Characteristic traveling-wave cascade optical amplifier. 2. The traveling wave cascade optical amplifier according to claim 1, wherein the optical filter is a diffraction grating formed on the same semiconductor substrate. 3. A traveling wave cascade optical amplifier according to claim 2, wherein the optical system is formed on the same semiconductor substrate. 4. A plurality of traveling wave type optical amplifiers for respectively amplifying optical signals, an optical system for connecting the plurality of traveling wave type optical amplifiers in a cascade, and a control circuit for performing gain control of the traveling wave type optical amplifiers. In the traveling-wave type cascaded optical amplifier including a plurality of traveling-wave type optical amplifiers, a plurality of traveling-wave type active layer stripes are formed on the same semiconductor substrate. The control circuit includes an optical filter inserted in an input system or an output system, and the control circuit is a part of a plurality of traveling wave 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 operate as a photodiode, and a means to monitor the output coupled to this stripe as a leaked light from the adjacent stripe. When traveling wave cascade optical amplifier characterized in that it comprises a means for controlling the gain of the optical amplifier by the monitor output.