JP2988106B2 - Optical receiving circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunable optical receiving circuit in an optical communication system.

[0002]

2. Description of the Related Art In a tunable optical receiving circuit, a tuning circuit is provided between a photodetector and an amplifier. At a resonance frequency of the tuning circuit, the load impedance of the photodetector increases, and the circuit noise of the optical receiving circuit increases. Is suppressed. Therefore, when receiving a high-frequency signal having a small ratio between a required band and a center frequency (fractional band), such as a wireless signal, a high receiving sensitivity can be realized by using the tunable optical receiving circuit. The circuit types of the tunable optical receiving circuit include a parallel tuned type, a transform tuned type, a mixed tuned type, and
A secondary bandpass tuning type and the like are known. Regarding these tunable optical receiving circuits, for example, "Tuned Front-End Design for Heterodyne Optical by Jacobsen et al.
Receivers ", Journal of Lightwave Technology vol.
7, No. 1, p. 105 (1989).

[0003]

In each of the above-mentioned tunable optical receiving circuits, an inductor is connected between the photodetector and the ground. Therefore, the load impedance of the photodetector becomes low for a signal having a very low frequency, and the level of the signal input to the amplifier becomes very low. For a DC signal, the output of the photodetector is equivalent to being directly short-circuited to the ground, and this DC signal is not input to the amplifier. Therefore, it is difficult for a conventional tunable optical receiver to receive a low-frequency signal (for example, a baseband signal such as a television signal or a digital signal, or a modulation signal having a very low carrier frequency) simultaneously with a high-frequency signal. Was.

An object of the present invention is to realize a tunable optical receiving circuit capable of receiving a high-frequency signal and a low-frequency signal simultaneously with high sensitivity.

[0005]

An optical receiving circuit according to the present invention comprises:
In a tunable optical receiving circuit in which the output of the photodetector and the ground are connected by at least one inductor (L1),
An impedance element having a sufficiently large impedance in a low frequency region is inserted in series with the inductor (L1).

In the optical receiving circuit according to the present invention, the output of the photodetector and the ground are at least one inductor (L1).
And a capacitor (C1) is inserted in series with the inductor (L1).

In the optical receiving circuit of the present invention, the output of the photodetector and the ground are at least one inductor (L1).
In a tunable optical receiving circuit in which a capacitor (C1) is inserted in series with the inductor (L1), the output of the photodetector and the ground are connected in series with a resistor (R1), an inductor (L2), It is characterized in that the connection is made by any one of the connected resistor (R1) and inductor (L2) and the connected resistor (R1) and inductor (L2) connected in parallel.

In the optical receiving circuit according to the present invention, the output of the photodetector and the ground are at least one inductor (L1).
In the tunable optical receiving circuit in which a capacitor (C1) is inserted in series with the inductor (L1), a resistor (R2) and an inductor (L3) are connected in series with the capacitor (C1). One of the resistor (R2) and the inductor (L3), and the resistor (R2) and the inductor (L3) connected in parallel are inserted.

Further, in the optical receiving circuit of the present invention, the output of the photodetector and the ground are at least one inductor (L).
In the tunable optical receiving circuit connected in 1), a resistor (R3) is inserted in series with the inductor (L1).

[0010]

According to the present invention, an impedance element, for example, a capacitor is inserted in series with the inductor connecting the photodetector and the ground. However, the capacitance of this capacitor is selected so that the impedance is sufficiently low at the frequency of the high-frequency signal, and conversely the impedance is sufficiently large near the frequency of the low-frequency signal.
Therefore, in the high frequency region, this capacitor can be almost neglected, and the reception characteristics equivalent to those of the conventional tunable optical receiver can be obtained for the high frequency signal. On the other hand, the load impedance of the photodetector in the low frequency region has a value sufficient to receive a low frequency signal. Therefore, according to the present invention, it is possible to realize a high-sensitivity tunable optical receiver capable of receiving a low-frequency signal simultaneously with a high-frequency signal.

[0011]

Next, the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing a configuration of a first embodiment of the optical receiving circuit of the present invention. FIG. 2 is a diagram showing the configuration of the first embodiment in more detail. FIG. 3 shows the frequency characteristic of the input-converted noise current density of the optical receiver of this embodiment. In this embodiment, at the same time as the radio signal having the frequency of 800 MHz, a carrier having a frequency of 1 MHz which is FSK (frequency shift keying) modulated with a digital signal having a bit rate of 2400 bps is received.

As shown in FIGS. 1 and 2, the present optical receiving circuit comprises a photodetector 4, a tuning circuit 6, and an amplifier 8. In this embodiment, the photodetector 4 is PI
An N photodiode 10 is used, and a bias voltage Vb of +10 V is applied. The amplifier 8 uses the FET 12 whose source is grounded. The gate voltage Vg is applied through the choke coil 42 and the resistor 62, and the drain voltage Vd is supplied through the choke coil 44 and the load resistor 64. . Note that the capacitor 30,
32 is a bypass capacitor.

The tuning circuit 6 is a so-called parallel type tuning circuit in which the capacitor 20 and the inductor 40 are connected in parallel with the output of the photodetector 4. Assuming that the capacitance of the capacitor 20 is C and the inductance of the inductor 40 is L, the resonance frequency f of this parallel tuning circuit is f = 1 / (2π√LC). In the case of this embodiment, L was 20 nH, and C was 2 pF including the capacitance of the photodetector 4 and the input capacitance of the amplifier 8. Therefore, the resonance frequency f of this tuning circuit was about 796 MHz. At this time, as shown in FIG. 3, the input-converted noise current density near 800 MHz was suppressed to a very low value of 5 pA / √Hz, and a radio signal having a frequency of 800 MHz could be received with high sensitivity.

The capacitor 22 inserted in series with the inductor 40 prevents the load impedance of the photodetector 4 from lowering in a low frequency region, and enables reception of a low frequency signal. The capacitance of this capacitor 22 is 1
00 pF, and the impedance at 800 MHz is a very low value of about 2Ω. Therefore, the capacitor 22 hardly affects the reception characteristics near 800 MHz. On the other hand, one of the capacitors 22
The impedance at MHz has a large value of about 1.6 kΩ. For this reason, the input-converted noise current density around 1 MHz is suppressed to 10 pA / √Hz or less, and the frequency 1
MHz carrier wave was also received with high sensitivity at the same time.

The resistor 60 inserted between the photodetector 4 and the ground is for bypassing a photocurrent which is a direct current. Since the resistance value of the resistor 60 is set to a high value of 2 kΩ, it hardly affects the reception characteristics of the radio signal and the low frequency carrier.

As a modification of the first embodiment, an inductor having a sufficiently large inductance in a low-frequency region instead of the resistor 60 for photocurrent bypass,
It is also possible to use either a resistor and an inductor connected in series or a resistor and an inductor connected in parallel. For example, by using an inductor having an inductance of 50 μH instead of the resistor 60, the photocurrent can be bypassed without substantially affecting the reception characteristics of the radio signal and the low-frequency carrier.

FIG. 4 is a diagram showing a second embodiment of the optical receiving circuit of the present invention. In the present embodiment, the gate voltage of the FET 12 is controlled by the resistor 62, the choke coil 42, and the bypass capacitor 30 via the inductor 40 and the capacitor 22.
And between the two. Further, a DC blocking capacitor 26 is inserted between the resistor 60 and the resonance circuit 6.
The resistance value of the resistor 62 is 10 kΩ, and the gate voltage can be supplied without substantially affecting the reception characteristics of the radio signal and the low frequency carrier. The capacitance of the capacitor 26 is 0.1 μF.
The impedance of 6 is almost negligible for radio signals and low frequency carriers. The frequency characteristic of the input-converted noise current density of the optical receiver according to the present embodiment is shown in FIG.
Are almost the same as those of the first embodiment shown in FIG.

FIG. 5 is a diagram showing a third embodiment of the optical receiving circuit of the present invention. This embodiment is different from the first embodiment in that a resistor 66 for a photocurrent bus path is inserted in parallel with the capacitor 22. The resistance value of the resistor 66 is set to 2 kΩ, and the photocurrent can be bypassed without substantially affecting the reception characteristics of the radio signal and the low frequency carrier.

Further, as a modification of the third embodiment, instead of the resistor 66 for bypassing the photocurrent, an inductor having a sufficiently large inductance in a low frequency region, a resistor and an inductor connected in series, or a parallel inductor It is also possible to use one of the resistor and the inductor connected to. For example, by using a 1 kΩ resistor and a 25 μH inductor connected in series in place of the resistor 66 in FIG. 5, the photocurrent can be bypassed without substantially affecting the reception characteristics of the radio signal and the low-frequency carrier. be able to. According to the present embodiment, an optical receiving circuit having substantially the same characteristics as the first embodiment can be realized.

FIG. 6 is a diagram showing a fourth embodiment of the optical receiving circuit of the present invention. FIG. 7 shows the frequency characteristics of the input-converted noise current density of the optical receiver of this embodiment.
In the present embodiment, a transform-type tuning circuit having two resonance frequencies is used, and radio signals of frequencies 800 MHz and 1500 MHz can be simultaneously received. Further, a digital signal having a bit rate of 2400 bps can be received.

The tuning circuit 6 composed of the inductors 46, 48, 50 and the capacitors 34, 36 in FIG. 6 has the same form as an equivalent circuit of a transformer (transformer), and is called a transform type tuning circuit. . In this embodiment, the inductances of the inductors 46, 48, and 50 were 27 nH, 0.1 nH, and 10 nH, respectively. The capacitances of the capacitors 34 and 36 were 0.8 pF and 2 pF, respectively. However, capacitor 3
The capacitance of 4 includes the capacitance of photodetector 4, and the capacitance of capacitor 36 includes the input capacitance of amplifier 8. This tuning circuit 6 has 81
It has two resonance frequencies of 0 MHz and 1450 MHz. Therefore, as shown in FIG. 7, the input-converted noise current density at around 800 MHz and 1500 MHz is suppressed to 8 pA / √Hz or less, and the frequency is 800 MHz.
Hz and 1500 MHz radio signals could be received simultaneously with high sensitivity.

In the present embodiment, a 75 Ω resistor 68 is inserted in series with the inductor 50. Accordingly, the load impedance of the photodetector 4 near DC was 75Ω, and the input-converted noise current density near DC was 20 pA / √Hz as shown in FIG. As a result, a digital signal having a bit rate of 2400 bps could be received simultaneously with the wireless signal.

In the present invention, various modifications other than the above-described embodiment are conceivable. For example, in the above-described embodiment, the common source type FET 12 is used as the amplifier 8, but another amplifier, for example, a transistor, a common gate type FET, or an integrated type amplifier may be used.

In the above embodiment, P is used as the photodetector 4.
Although the IN photodiode 10 was used, other photodetectors,
For example, a photodiode, an avalanche photodiode, a phototransistor, or the like can be used.

As the tuning circuit 6, parallel tuning circuits are used in the first, second and third embodiments, and a transform tuning circuit is used in the fourth embodiment. However, other tuning circuits, for example, It is also possible to use a mixed tuning circuit, a third-order band-pass tuning circuit, or the like.

Further, in the first, second and third embodiments, by using the capacitance of the photodetector 4 or the input capacitance of the amplifier 8, the tuning circuit 6
Can be omitted. Similarly, in the fourth embodiment, the capacitor 34 can be omitted by using the capacitance of the photodetector 4, and the capacitor 36 can be omitted by using the input capacitance of the amplifier 8. Further, in the fourth embodiment, the inductor 46 can be omitted by using the output inductance of the photodetector 4, and the inductor 48 can be omitted by using the input inductance of the amplifier 8.

[0027]

As described above, according to the present invention, in a tuning type optical receiving circuit, a tuning inductor and a tuning inductor are connected in series.
In the high frequency region, low impedance
Now insert a capacitor with a capacitance value that indicates high impedance
Therefore, it is possible to receive a high-frequency signal and a low-frequency signal simultaneously with high sensitivity without affecting the tuning characteristics .

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing the embodiment of FIG. 1 in detail.

FIG. 3 is a diagram showing a frequency characteristic of an input-converted noise current density in the embodiment of FIG. 1;

FIG. 4 is a circuit diagram showing a second embodiment of the present invention.

FIG. 5 is a circuit diagram showing a third embodiment of the present invention.

FIG. 6 is a circuit diagram showing a fourth embodiment of the present invention.

FIG. 7 is a diagram showing a frequency characteristic of an input-converted noise current density in the embodiment of FIG. 6;

[Explanation of symbols]

2 signal light 4 photodetector 6 tuning circuit 8 amplifier 10 PIN photodiode 12 FET 20, 22, 24, 26, 28, 30, 32, 34, 3
6 Capacitor 40, 42, 44, 46, 48, 50 Inductor 60, 62, 64, 66, 68 Resistance

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ identification symbol FI H04B 10/14 10/26 10/28 (58) Investigated field (Int.Cl. ⁶ , DB name) H03J 3/22 H04B 10 / 00-10/28

Claims (5)

(57) [Claims] In a tunable optical receiving circuit in which an output of a photodetector and a ground are connected by at least one inductor (L1), an impedance element having a sufficiently large impedance in a low frequency region is connected to the inductor (L1).
An optical receiving circuit, which is inserted in series with 1). 2. A tunable optical receiving circuit in which an output of a photodetector and a ground are connected by at least one inductor (L1), wherein a capacitor (C1) is inserted in series with the inductor (L1). Optical receiving circuit. 3. The output of the photodetector and ground are connected to a resistor (R1), an inductor (L2), a resistor (R1) and an inductor (L2) connected in series, and a resistor (R1) connected in parallel. 3. The optical receiving circuit according to claim 2, wherein the optical receiving circuit is connected to any one of the inductor and the inductor. 4. A resistor (R2) and an inductor (L3) in parallel with the capacitor (C1), a resistor (R2) and an inductor (L3) connected in series, and a resistor (R2) and an inductor connected in parallel. 3. The optical receiving circuit according to claim 2, wherein any one of (L3) is inserted. 5. A tunable optical receiving circuit in which an output of a photodetector and a ground are connected by at least one inductor (L1), wherein a resistor (R3) is inserted in series with the inductor (L1). Optical receiving circuit.