JP2547039B2 - Light receiving device for optical communication - Google Patents

Description

The present invention relates to a light receiving device for optical communication used in a coherent optical communication heterodyne detection system, which is intended for widening a band without deteriorating noise characteristics, and which receives an optical signal. In an LC tuning type optical communication light receiving device including an element, an amplifier for inputting an output signal of the light receiving element, and an inductance, a bootstrap type configuration for returning an output signal of the amplifier through an impedance element And

[Industrial applications]

The present invention relates to a light receiving device for optical communication used in a coherent optical communication heterodyne detection system.

By improving the characteristics of the semiconductor laser by the coherent optical communication system, it is possible to easily transmit high-speed data of several Gb / S or more, and accordingly, there is a demand for widening the band of the light receiving device.

[Conventional technology]

The light receiving device used for the coherent optical communication heterodyne detection system is required to have a band of 3 to 5 times the data bit rate. FIG. 5 is an explanatory diagram of frequency characteristics, in which the vertical axis represents the spectrum intensity and the horizontal axis represents the frequency. In FIG. Shown, fi is the intermediate frequency. The light receiving device when such a modulation method is applied is
As shown by the dotted line, three times the baseband signal band was required. Further, (b) of the figure shows the characteristics in the FSK dual filter system, which is modulated to frequencies f1 and f2. Therefore, the light receiving device when such a system is applied is In addition, 5 of the baseband signal band
Twice was needed.

However, in practice, it is sufficient if there are bands that pass the spectrum centered at the intermediate frequency fi or the spectrum centered at the frequencies f1 and f2, respectively, so that as the light receiving device, ASK, PSK, DPSK, etc. When applying the modulation method,
If the FSK dual filter method is applied to the band shown by the dotted line in (c) of the figure, the band shown by the dotted line in (d) of the diagram is sufficient.

From such a point, an LC tuning type light receiving device as shown in FIG. 6 has been proposed. In the figure, 41 is a light receiving element such as a PIN diode or avalanche photodiode (APD), 42 is an amplifier including a field effect transistor,
43 is an inductance, 44 is a resistance, and C _D is a junction capacitance of the light receiving element 41.

An inductance 43, Sadamari junction capacitance C _D and the tuning frequency by the input capacitance of the amplifier 42 of the light receiving element 41, the inductance 43 L, the combined capacitance of the junction capacitance C _D of the input capacitance and the light receiving element 41 of the amplifier 42 C _Assuming that _T is _T and resistor 44 is R _B , the gain frequency characteristic of this LC tuning type photo detector is 7th.
It will be as shown in the figure. That is, the center frequency f ₀ is The 3 dB band B ₃ has a characteristic of B ₃ = 1 / 2πR _B C _T. Therefore, in the case of FIG. 5 (c), the inductance 43 and the like are set so that f ₀ = fi, and the resistor 44 is set so as to obtain a desired band.
(R _B ) will be set. Further, in the case of FIG. 5 (d), the inductance 43 and the like are set so that the intermediate frequency between f1 and f2 becomes the center frequency f _0, and the resistor 44 (R _B ) is set so as to obtain the desired band. Will be done.

Such an LC tuning type photo detector has a bandwidth of 10 MHz.
In the case of not more than about this, the characteristics close to the shot noise limit can be obtained. In order to further expand the band, a transimpedance type light receiving circuit as shown in FIG. 8 is known. In the figure, 51 is a light receiving element, 52 is an amplifier, 53 is an inductance, 54 is a feedback resistor, 55 is a bias resistor, 56 is a wide band low noise field effect transistor which constitutes a pre-amplifier, 57
Is a post-stage amplifier, and 58 is an output terminal.

Although the amplifier 52 shows a case of a multi-stage configuration,
It is of course possible to use a single stage, in which the gain of the amplifier 52 is A, the bias resistor 55 is R _B , the inductance 53 is L, the feedback resistor 54 is R _f , the junction capacitance of the light receiving element 51 and the amplifier 52.
If the combined capacitance such as the input capacitance of is C _T , the equivalent circuit shown in FIG. 9 is obtained. i _s is a current source in which a current flows in response to an optical signal incident on the light receiving element 11, and a tuning circuit is constituted by the combined capacitance C _T and the inductance L.

From the equivalent circuit of FIG. 9, the transimpedance is
| Z _T | is However, The combined capacitance C _T is C _T = C _D + C _S + C _GS + C _GD (1 + g _{m RL} ) (2) when the field-effect transistor is used in the source-grounded form. Note that C _D is the junction capacitance of the light receiving element 11, C _S is the stray capacitance, C _GS is the gate-source capacitance of the field effect transistor, C _GD is the gate-drain capacitance of the field effect transistor, and g _m is the field effect transistor. , R _L is the load resistance.

Bandwidth B _if is Therefore, the band is expanded by the gain A times.

[Problems to be solved by the invention]

In the coherent optical communication heterodyne detection system, high-speed data transmission of several Gb / S or more is possible, so a band of the light receiving device corresponding to it is required. In order to obtain such a wide band characteristic, it is conceivable to reduce the resistance 44 (R _B ) in the configuration of the conventional example shown in FIG. However, if the resistor 44 (R _B ) is made small, the reception sensitivity will decrease and the thermal noise will increase. Therefore, it is difficult to realize a configuration suitable for high-speed data transmission of several Gb / S or more. there were.

Further, in the conventional example shown in FIG. 8, the open loop gain must be increased in order to widen the band.
Considering the gain per field-effect transistor that constitutes 52, it is necessary to adopt an amplification configuration of several stages at present.
As a result, the return path becomes long, the stray capacitance and stray inductance increase, and the high-frequency characteristics deteriorate.

An object of the present invention is to broaden the band without deteriorating the noise characteristic.

[Means for solving problems]

The light receiving device for optical communication of the present invention is an LC tuning type device having a wide band, which will be described with reference to FIG. An LC tuning type light receiving device for optical communication, comprising a light receiving element 1 for inputting an optical signal, an amplifier 2 including a field effect transistor for inputting an output signal of the light receiving element 1, and an inductance 3, The output signal 2 is fed back through the impedance element 4 such as a capacitor and has a bootstrap type configuration.

[Action]

Since the output signal of the amplifier 2 of the LC tuning type optical communication light receiving device is fed back through the impedance element 4 to form the bootstrap type, the combined capacitance C _T can be made small, so that the band can be expanded. . Further, since the gain may be 1, only one transistor is needed and the feedback path can be shortened.

〔Example〕

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

FIG. 2 is a circuit diagram of a main part of one embodiment of the present invention, in which 21 is a light receiving element, 22 is a field effect transistor constituting an amplifier, 23 is an inductance, 24 is a feedback capacitor, and 2 is a feedback capacitor.
5, 26 are resistors and 27 is an output terminal.

The output signal of the light receiving element 21 is connected so as to be applied to the gate of the field effect transistor 22.
The drain of 22 is grounded, the output terminal 27 is connected to the source, and it is connected to the -V power source through the resistor 26.
Connection is made from 27 through the capacitor 24 so as to return to the light receiving element 21 side.

Figure 3 shows an equivalent circuit of FIG. 2, i _s the current source current flows in response to the optical signal incident on the light receiving element 21, C _D is the junction capacitance of the light receiving element 21, L is the inductance 23, R _B is a bias resistor 25, C ₁ is C _GD + (1-A) C _GS + C _S , g _m V _GS is a current source through which a current corresponding to the mutual conductance g _m flows,
C ₂ is (1-1 / A) C _GS + C _DS , R _S is the resistor 2 connected to the source
6, r _DS is the drain-source resistance, C _GS is the gate-source capacitance, C _GD is the gate-drain capacitance, and C _S is the stray capacitance.

Transimpedance in this equivalent circuit | Z _T |
Is However, Q = 2πf _if R _B C _T C _T = C _GS + (1-A) C _GS + C _S + (1-A) C _D Becomes The transimpedance | Z _T | is the same as in the case of the normal LC tuning type, but since the combined capacitance C _T is reduced by the bootstrap type, the band can be expanded by it.

Since the gain of the field effect transistor 22 of this embodiment is 1, only one field effect transistor is required, and therefore the feedback path can be shortened. For that reason,
Stray capacitance and stray inductance can be reduced. Therefore, it becomes possible to improve the high frequency characteristics.
A wide band low noise amplifier can be connected to the output terminal 27 to amplify the light reception output signal.

FIG. 4 is a circuit diagram of a main part of another embodiment of the present invention, showing a case where the present invention is applied to a dual balance type light receiving device. In the figure, 31a and 31b are light receiving elements, 32 is a field effect transistor forming an amplifier, 33 is an inductance, and 34a and 34b.
Is a condenser for feedback, 35a, 35b and 36 are resistors, 38 is a bias resistor, 37 is an output terminal, and 39 is an optical coupler.

The received optical signal and the locally oscillated light are mixed by the optical coupler 39 into two output lights, and the light receiving elements 31a and 31 are respectively provided.
It is incident on b. The light receiving elements 31a and 31b are connected in series, and the connection point is connected to the gate of the field effect transistor 32. The output lights of the optical coupler 39 have a phase difference of π, and the output signals of the two light receiving elements 31a and 31b are differentially combined in the field effect transistor 32.
By equalizing the response speeds of the two light receiving elements 31a and 31b, an output signal having a level twice that of the output signal from one light receiving element can be obtained at the output terminal 37. Since the local oscillation light intensity noise has a correlation, it becomes zero in principle by taking the differential combination of the output signals of the two light receiving elements 31a and 31b, and the heterodyne detection output signal is obtained.

In addition, the junction capacitance of the light receiving elements 31a and 31b, the gate-drain capacitance of the field effect transistor 32, and the inductance 33
LC tuning type is composed of
A bootstrap type is formed by feeding back the output signal of 32 through the capacitors 34a and 34b, and the band can be expanded by reducing the combined capacitance C _T as in the embodiment shown in FIG. .

The input conversion noise current when the bit rate is 4 Gb / S will be described. The bias resistor 44 (R _B ) in the conventional example shown in FIG. 6 is set to 1 KΩ, the gain A of the amplifier 52 in the conventional example shown in FIG. 8 is 20 dB, and the bias resistor 55 (R _B ) is (A + 1) / 2πC _T (2B _r ) (B _r is the bit rate)
In addition, the bias resistance 25 (R _B ) in the embodiment of the present invention shown in FIG. 2 is 1 KΩ, C _D = 0.3 pF, C _GS = 0.5 pF, C _DS
= 0.05pF, C _S = 0.1pF, g _m = 25mS (Siemens), R _L = 1
When it is set to 50Ω, the input converted noise current is as follows in the conventional example shown in FIG. In the conventional example shown in FIG. In the embodiment of the present invention shown in FIG. 2, Became.

Therefore, according to the configuration of the embodiment of the present invention, the input conversion noise current is smaller than that of the conventional example shown in FIG. 6 and the band can be expanded, and compared with the conventional example shown in FIG. The input equivalent noise current is almost the same, but the combined capacitance C _T
Can be reduced to widen the band and shorten the return path, thereby improving the high frequency characteristics.

〔The invention's effect〕

As described above, according to the present invention, in the LC tuning type light receiving device, the output signal of the amplifier 2 is changed to the impedance element 4.
The amplifier 2 is configured by one field-effect transistor by reducing the combined capacitance C _T to expand the band without deteriorating the noise characteristics by returning the signal via the circuit. Therefore, the return path is shortened, the high frequency characteristics are improved, and the optical signal in the band of several Gb / S or more can be stably received.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of the present invention, FIG. 2 is a circuit diagram of an essential part of an embodiment of the present invention, FIG. 3 is an equivalent circuit of FIG. 2, and FIG. 4 is another embodiment of the present invention. Main circuit diagram, FIG. 5 (a)-
(D) is an explanatory view of frequency characteristics, FIG. 6 is a circuit diagram of a main part of a conventional example, FIG. 7 is an explanatory view of gain frequency characteristics, FIG. 8 is a transimpedance type light receiving circuit of a conventional example, and FIG. The equivalent circuit of the figure is shown. 1, 21, 31a, 31b are light receiving elements, 2 are amplifiers, 22 and 32 are field effect transistors, 3, 23 and 33 are inductances, 4 is an impedance element for feedback, 24, 34a and 34b are capacitors for feedback, Reference numerals 25 and 38 are bias resistors, and 39 is an optical coupler.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location H04B 10/152 10/26 (72) Inventor Takao Naito 1015 Uedoda, Nakahara-ku, Kawasaki-shi, Kanagawa Fujitsu Within the corporation

Claims (1)

(57) [Claims] 1. An LC-tuning type optical communication comprising a light receiving element (1) for receiving an optical signal, an amplifier (2) for inputting an output signal of the light receiving element (1), and an inductance (3). The light receiving device for optical communication is characterized in that it has a bootstrap type configuration in which an output signal of the amplifier (2) is fed back through an impedance element (4).