JP3187457B2 - Optical transmitting / receiving element and optical transmitting / receiving device using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitting / receiving element used in technical fields such as optical communication or optical measurement and optical signal processing, and an optical transmitting / receiving apparatus using this element.

[0002]

2. Description of the Related Art In optical communication equipment and measuring equipment, the same transmission line (optical fiber, spatial beam, etc.) is used as a dual way (two directions), and transmission and reception are performed in a time-division manner so as to be shared for transmission and reception. Often used by switching to. FIG.
FIG. 1 shows an embodiment of a conventional optical transceiver.

Since the light emitting element 19 for transmission and the light receiving element 18 for reception use different elements, the optical signal from the transmission optical fiber 12 is once converted into parallel light by the lens 11 and then converted to parallel light. The light is split by a beam splitter 16 or an optical splitter 16 such as a directional coupler is inserted into a transmission path and split, and one is split to a light emitting element 19 and the other is split to a light receiving element 18 by a light receiving optical system 17. It needed to be focused.

In order to stabilize the signal and improve the reliability, a light emitting output monitoring light receiver 9 is separately provided, and this light receiving device 9 is combined with a part of the light output of the light emitting element 19 to be used for monitoring. After the signal is amplified by the amplifier 10, the transmission output is stabilized by feeding back the signal to the light emission drive source 8.

[0005] Since the light emitting element and the light receiving element are separate, an optical element for branching the optical path is required, and there has been a problem that the configuration is complicated. When transmission and reception are performed at the same wavelength, they cannot be separated by a wavelength filter or the like, and the transmission signal and the reception signal must generate a loss of 3 dB due to the branching means.

It is generally known that a light emitting element used for optical communication and measurement has reversibility and can be used as a light receiving element by allowing light to enter from a light emitting section. Attempts have been made to simplify the structure of not only the element but also the optical system by using the same material for the light emitting element and the light receiving element utilizing this characteristic.

However, when a semiconductor laser is used as a light receiving element, for example, the performance of the optical transmitting and receiving element is far from a single light receiving element in terms of dark current, parasitic capacitance, and conversion efficiency. Did not reach. For example,
In the case of a photodetector using the ternary substance InGaAs, the dark current is about several nA to about 100 nA and the parasitic capacitance is several pF, whereas in the case of a photodetector using a semiconductor laser, the bias voltage is At 2 V, the dark current is several μA, and the parasitic capacitance is 50
It was around pF, and it was difficult to provide for practical use.

[0008]

SUMMARY OF THE INVENTION The object of the present invention is, firstly, to use, as a light receiving element, the same element as a semiconductor light emitting element (laser diode) which has not been successful until now. Second, a transmitter and a receiver are configured by a single element for use in a communication or measurement system by using the element having the function of emitting and receiving light. It has a function to stabilize the transmission level. It has a function to monitor the transmission level.
An object of the present invention is to realize an optical transmitting and receiving apparatus characterized by the above.

[0009]

SUMMARY OF THE INVENTION Two electrodes are provided in the length direction of an active layer of a semiconductor light emitting element, and the length of a first electrode (front electrode), which also functions as a light receiving element, is extremely short. By reducing the capacitance and dark current, a two-electrode light-emitting and light-receiving element, which has not been put to practical use, has been realized. In addition, a combination of two electrodes and a control system can be adopted.

[0010]

Some preliminary considerations will be given to realize these elements and devices. 1) When a semiconductor laser is considered as a light receiver, the area of the light receiving portion must be small (1 μm × 0.1 μm) in order to increase the light emission efficiency.
m) can be used in common for light reception by a coupling system as a light emitting element, so that there is no great disadvantage.

2) It is necessary to suppress the coupling efficiency between the semiconductor laser and the transmission optical fiber to about 2 to 3 dB. However, since the light emission and the light reception have a common optical path, a branch coupler is not required. In the case of light emission, 3d by an optical branching coupler
B loss can be reduced. Even in the case of light reception, 2-3 dB of optical coupling loss is offset by 3 dB of branch loss.

3) However, in order to improve the parasitic capacitance and dark current, in order to obtain a predetermined luminous efficiency and light output during light emission, the semiconductor laser is usually designed to have a length of about 300 μm. Cannot be shortened. However, if the length of the photoelectric conversion portion can be reduced, both the parasitic capacitance and the dark current can be reduced substantially in proportion to the reduced length (active layer length).

4) When the active layer of the semiconductor laser is not excited, light having a wavelength equal to or less than the emission wavelength is used.
It is known that the absorption coefficient is as large as several thousand / cm. (Reference "Absorption and electroabsor
ption spectra of an In _1-X Ga _X P _1-Y As _Y / InP doubl
e heterostructure "K. Satzke and G. Weiser, J. Appl.
Phys. 63 (11), 1 June 1988, PP5485-5490).

For example, confinement coefficient 0.2, absorption coefficient 3
When light is incident from the end face of the active layer of about 000 / cm, the length of the active layer is about 50 μm, and about 95% of the input light, 30 μm
In this case, about 85% of the input light is absorbed, so that even if the active layer length is longer, the light receiving sensitivity hardly changes. Rather, the generated power is used to excite non-photosensitive portions through the electrodes, reducing the power taken out.

Therefore, the electrodes of the semiconductor laser are at least electrodes on the signal light input / output side (also referred to as first electrodes or front electrodes) and electrodes opposite to the signal light input / output side (second electrodes). Or, it is also called a rear electrode.)
The length of the first electrode is 30 μm, for example, when the light received from the end face is used. When the first electrode is used as a light emitting device (light emitting element), a current flows through the first electrode and the second electrode, and the light receiving device (light receiving device) When used as an element, only the first electrode is used, and the second electrode is kept at no bias or almost at ground bias. By doing so, the parasitic capacitance and dark current can be reduced to about 1/10 or less as compared with the related art. FIG. 2 shows an equivalent circuit of the semiconductor laser.

5) Since the maximum receiving frequency f when the load resistance R is 50Ω is given by f = １ ／ πRC, the conventional technology has a parasitic capacitance of about 60 MHz when the parasitic capacitance is 50 pF.
In contrast, in the present invention, it can be seen that the parasitic capacitance is increased by a factor of 10 to about 600 MHz by reducing the parasitic capacitance to 1/10 of 5 pF. In addition, a reduction in sensitivity due to absorption of electromotive force is prevented.

6) On the other hand, when a semiconductor laser having two electrodes is used as a light emitting device, intensity modulation can be performed by keeping the current flowing to the front electrode constant and changing the current flowing to the rear electrode. . It has been reported that the terminal voltage changes when the amount of light in the active layer of the light emitting element changes while current is injected into the semiconductor laser (see “Terminal voltage change of self-coupled semiconductor laser” (Electronics ITE Technical Report OQE80-8
3) Yoshiki Mitsuhashi, Junichi Shimada, Shuichi Mitsuka).

Taking a hint from this report, when used as a light-emitting device, the present inventors apply intensity modulation by changing only the current of the rear electrode, and apply a constant current to the front electrode. It has been found that by detecting the terminal voltage, a voltage change corresponding to the change in the light amount of the electrode portion can be detected, and the light emission amount can be monitored. This eliminates the need to install a separate monitor light receiver for the amount of emitted light, simplifying the configuration,
The configuration of the present invention that can be economical can be produced.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the configuration of an optical transceiver according to the present invention, and FIG. 2 shows an equivalent circuit using the semiconductor laser.
One embodiment of the configuration of the optical transceiver according to the present invention is shown in FIGS. Optical transmitting / receiving element (semiconductor laser) of the present invention
Means that the electrode is a front electrode (first electrode) 3 in the waveguide direction.
And a rear electrode (second electrode) 4. First
The electrode 3 and the second electrode 4 are bonded to and separated from the semiconductor portion 1 having a double hetero structure including the active layer 2. The front electrode 3 is, for example, about 30 μm. In 50μm at most
If it is longer , parasitic capacitance and dark current will increase.
Come, that for receiving the more harm than good has been found from the experimental results. The electrodes 3 and 4 having a crank shape are spaces for wire bonding. A light emission driving source 8 is connected to the rear electrode 4, and the rear electrode 4, the light emission driving source 8, the light emitting output monitoring light receiver 9, and the monitoring amplifier 10 are provided.
, An automatic light output control (APC) circuit is assembled.
The front electrode 3 is connected to the transmission modulation signal similarly output from the light emission drive source 8 at the time of transmission by switching of the switch 5, applies a reverse voltage at the time of reception, and receives a light reception signal processing circuit via the light reception signal amplifier 6. 7 is connected.

In this case, since the front electrode 3 is shorter than the rear electrode 4, no modulation signal is applied to the front electrode 3 and a constant current is applied. Does not drop. If a current equal to or greater than the total oscillation threshold value × the length of the front electrode 3 / the total electrode length is passed through the front electrode 3, a sufficient output during oscillation can be obtained. At the time of reception, the rear electrode 4 maintains no bias or a constant bias near the ground (also referred to as a ground bias) by the switch 5 so as not to hinder signal reception.

Thus, at the time of light reception, the active layer 2 functions as a light receiver, and the light received from the transmission optical fiber 12 is converted into an electric signal by the same optical coupling means 11 at the time of light emission with almost the same coupling efficiency as at the time of light emission. I do. This electric signal is amplified to a predetermined level by the light receiving signal amplifier 6 and received by the light receiving signal processing circuit 7. At the time of transmission, a constant level bias current may be applied to the front electrode 3 instead of the modulation signal, and the effect remains unchanged.

When the light emission output is not required much, laser oscillation can be performed without supplying a current to the front electrode 3. In this case, the switch 5 for switching between the light emission signal and the light reception signal is not required, and the monitor signal is processed in the same manner as the light reception signal, so that the circuit is simplified.

In the case of the optical transmitting / receiving apparatus according to the present invention, as shown in FIG. 4, during transmission, a bias current supply (constant current source) 13 is connected to the front electrode 3 by the switch 5 and the rear electrode 4 emits light. Ru is connected to the drive source 8. At this time, if the amount of light in the active layer 2 changes due to the modulation at the rear electrode 4,
The terminal voltage of the front electrode 3 changes. The change in the terminal voltage is transmitted from the light receiving signal amplifier 6 through the switch 5 to the light emission driving source 8.
, And by driving the rear electrode 4, an automatic light output control (APC) circuit can be assembled. Since the detection of the terminal voltage of the front electrode 3 includes a constant voltage due to the bias current, the switch 5 cuts the DC component,
It is assumed that only a voltage change corresponding to a change in the amount of modulated light is connected to the light receiving signal amplifier 6.

At the time of reception, similarly to the embodiment of FIG. 3, the front electrode 3 of the optical transmitting / receiving element is connected to the light receiving signal processing circuit 7 via the light receiving signal amplifier 6, and the rear electrode 4 of the optical transmitting / receiving element is connected. Are connected so as to be in a bias-free potential state. When the light emission output does not need to be very large, laser oscillation can be performed without passing a current to the front electrode 3. In this case, the switch 5 for switching between the light emission signal and the light reception signal is not required, and the monitor signal can be processed in the same manner as the light reception signal, so that the circuit can be simplified. It goes without saying that the optical coupling means 14 in this embodiment can be realized without using a lens or the like and using a spherical fiber lens.

[0025]

The electric / optical conversion element used in the optical communication device and the measurement / measurement device generally has reversibility, and can be used as a light receiving element by receiving light from a light emitting section. According to the present invention, by dividing the electrode of the element into two, the element can be used for both light emission and light reception, and can exhibit characteristics comparable to those of a single light receiving element at the time of light reception. As a result of using the same light-emitting element and light-receiving element, not only the number of elements but also the optical system can be simplified, and the loss due to division is eliminated, so that the loss during light emission can be reduced by 3 dB. Since there is no need to insert a light branching means in the middle of the connection with the optical fiber, it is also possible to directly connect the optical fiber to the spherically processed fiber, and the optical system is further simplified. Further, at the time of light emission, the light-emitting signal can be monitored by the light-receiving electrode, so that the light-receiving element for monitoring and the optical system for it can be omitted, and a great economical effect can be expected.

[Brief description of the drawings]

FIG. 1 shows an embodiment of a configuration relating to an optical transmitting / receiving element of the present invention.

FIG. 2 shows an equivalent circuit of a semiconductor laser.

FIGS. 3 and 4 show an embodiment of the configuration relating to the optical transmitting / receiving apparatus of the present invention.

FIG. 5 shows an embodiment of the configuration of the prior art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor part 2 Active layer 3 1st electrode (front electrode) 4 2nd electrode (rear electrode) 5 Switch 6 Light reception signal amplifier 7 Light reception signal processing circuit 8 Light emission drive source 9 Light emission output monitor light receiver 10 For monitor Amplifier 11 Lens 12 Transmission optical fiber 13 Bias current supply device 14 Optical coupling means (Spherical fiber lens) 15 Light quantity change detector 16 Optical splitting means (beam splitter) 17 Light receiving optical system 18 Light receiving element 19 Light emitting element

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H04B 10/06 10/14 10/26 10/28 (56) References JP-A-59-125660 (JP, A) JP-A Sho JP-A-61-187286 (JP, A) JP-A-61-191091 (JP, A) JP-A-62-81784 (JP, A) JP-A-62-285484 (JP, A) JP-A-2-299282 (JP, A A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 H01L 31/00-31/20 H01L 33/00 H04B 10/00-10/30

Claims (3)

(57) [Claims] 1. A semiconductor portion of the double hetero structure including an active layer (2) and (1), the signal light output joined to the semiconductor unit
A first electrode (3) arranged on the side for extracting a received signal and the first electrode joined to the semiconductor portion are connected to each other;
And a second electrode disposed Isolated waveguide direction (4), the length of the waveguide direction on the active layer of the first electrode is shorter than the second electrode, its length light emitting and receiving elements you wherein a is 50μm or less. [2 claim] and the light receiving element according to claim 1, wherein, the light emission driving source that outputs a driving signal of the light receiving element (8), of the light emitting and receiving elements when using the light emitting and receiving elements as the light emitter the first electrode (3) and a second electrode (4) both connected to the light emission driving source, the light-receiving signal processing circuit said first electrode when using the light emitting and receiving elements as the photodetector (7) connected to, and, switch (5) for connecting said second electrode to the non-bias potential state and a light receiving device provided with a. 3. A light receiving device according to claim 1, the light transmission
A light-emitting drive source (8) for outputting a drive signal for a receiving element; and an optical drive when the optical transmitting / receiving element is used as a light emitting device.
Connect the first electrode (3) of the receiving element to the constant current source (13)
And the second electrode (4) of the optical transmitting and receiving element is
Connected to the light emission driving source, when using the light emitting and receiving elements as the photodetector connects the first electrodes on the light-receiving signal processing circuit (7), and no bias current to the second electrodes < br /> position switch that connects to the state (5), the first
Light amount change detector consisting of means for detecting the terminal voltage of the electrode (15) and a light receiving device provided with a.