# US 7,609,982 B2 · RZ-DPSK optical receiver circuit

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- Number: US 7,609,982 B2
- Document ID: US7609982B2
- Status: Granted
- Kind: B2
- Application number: 11/270,655
- Filed: 2005-11-10
- Published: 2007-03-01
- Granted: 2009-10-27
- Earliest claimed: 2005-08-25
- Assignees: Fujitsu Limited
- Inventors: Tadashi Ikeuchi
- Web page: https://patentgrep.com/patent/US7609982B2

## CPC

- H04B 10/677: {for differentially modulated signal, e.g. DPSK signals}
- H04B: TRANSMISSION

## Abstract

A received RZ-DPSK signal is guided to an interferometer. The interferometer comprises a 1-bit delaying element in an upper arm. A balanced photodiode converts a pair of optical signals output from the interferometer into electrical signal. A high-pass filter, which has a cut-off frequency equal or approximately equal to the symbol rate of transmission data, filters the signal output from the balanced photodiode. A BPSK demodulator circuit demodulates the output signal of the high-pass filter.

## Claims

### Claim 1

A RZ-DPSK optical receiver circuit comprising:
an interferometer, comprising a 1-symbol delaying element in one of paired optical paths, for transmitting an input RZ-DPSK signal via the paired optical paths;
a photo detector circuit for converting an optical signal output from said interferometer into an electrical signal;
a high-pass filter with a cut-off frequency which is equal or approximately equal to the symbol rate of the RZ-DPSK signal, for filtering an output signal of said photo detector circuit; and
a BPSK demodulator circuit for demodulating an output signal of said high-pass filter, wherein
said interferometer outputs a pair of complementary optical signals, and
said photo detector circuit comprises a pair of photodiodes for respectively converting the pair of the optical signals into a pair of electrical signals and outputs a differential signal of the pair of electrical signals acquired by the pair of photodiodes, and
said high-pass filter acquires a BPSK signal from the differential signal, and
said BPSK demodulator circuit demodulates the BPSK signal.

### Claim 2 (depends on 1)

The RZ-DPSK optical receiver circuit according to claim 1, further comprising
an amplifier, provided between said photo detector circuit and said high-pass filter, for amplifying an output signal of said photo detector circuit.

### Claim 3 (depends on 1)

The RZ-DPSK optical receiver circuit according to claim 1, wherein the BPSK demodulator circuit comprises:
an oscillator circuit for generating a pair of clock signals, which are orthogonal to each other;
a pair of multipliers for multiplying the output signal of said high-pass filter and the pair of clock signals; and
a control circuit for controlling the frequency of the clock signal generated by the oscillator circuit based on the outputs of the pair of multipliers.

### Claim 4 (depends on 1)

The RZ-DPSK optical receiver circuit according to claim 1, further comprising
a determination circuit for determining a logical value of data in response to voltage of the output signal of said photo detector circuit, using the clock signal recovered by said BPSK demodulator circuit.

### Claim 5 (depends on 4)

The RZ-DPSK optical receiver circuit according to claim 4, further comprising
a low-pass filter, provided in the preceding stage of said determination circuit, for filtering the output signal of said photo detector circuit.

### Claim 6 (depends on 4)

The RZ-DPSK optical receiver circuit according to claim 4, further comprising:
a control circuit for generating a control signal in response to the comparison result between data acquired from said BPSK demodulator circuit and data acquired by said determination circuit; and
an adjustment circuit for adjusting a phase of a clock signal provided from said BPSK demodulator circuit to said determination circuit in accordance with the control signal.

### Claim 7 (depends on 4)

The RZ-DPSK optical receiver circuit according to claim 4, further comprising:
a limiter amplifier, provided in the preceding stage of said determination circuit, for amplifying an output signal of said photo detector circuit; and
a control circuit for generating a control signal in response to a comparison result between data acquired from said BPSK demodulator circuit and data acquired from said determination circuit, wherein
the limiter amplifier amplifies the output signal of said photo detector circuit in accordance with the control signal.

### Claim 8

A RZ-DPSK optical receiver circuit, comprising:
an interferometer, comprising a 1-symbol delaying element in one of paired optical paths, for transmitting an input RZ-DPSK signal via the paired optical paths;
a photo detector circuit for converting an optical signal output from said interferometer into an electrical signal;
removing means for removing frequency component lower than the symbol rate of the RZ-DPSK optical signal from the output signal of said photo detector circuit; and
a BPSK demodulator circuit for demodulating the signal, whose frequency component lower than the symbol rate being removed by said removing means, wherein
said interferometer outputs a pair of complementary optical signals, and
said photo detector circuit comprises a pair of photodiodes for respectively converting the pair of the optical signals into a pair of electrical signals and outputs a differential signal of the pair of electrical signals acquired by the pair of photodiodes, and
said removing means acquires a BPSK signal from the differential signal, and
said BPSK demodulator circuit demodulates the BPSK signal.

### Claim 9 (depends on 8)

The RZ-DPSK optical receiver circuit according to claim 8, wherein
said removing means is an amplifier, which has zero or sufficiently small gain in the frequency range lower than the symbol rate, for amplifying the output signal of said photo detector circuit.

### Claim 10

A RZ-DQPSK optical receiver circuit, comprising:
an optical splitter for generating first and second RZ-DQPSK signals by splitting a RZ-DQPSK signal;
a first interferometer, comprising a 1-symbol delaying element in one of a pair of optical paths and a phase shifter for shifting the phase of a carrier wave of the first RZ-DQPSK signal by π/4 in the other optical path of the pair, for transmitting the first RZ-DQPSK signal via the pair of the optical paths;
a first photo detector circuit for converting an optical signal output from said first interferometer into an electrical signal;
a first high-pass filter with a cut-off frequency which is equal or approximately equal to the symbol rate of the RZ-DQPSK optical signal, for filtering the output signal of said first photo detector circuit;
a first BPSK demodulator circuit for demodulating the output signal of said first high-pass filter;
a second interferometer, comprising a 1-symbol delaying element in one of a pair of optical paths and a phase shifter for shifting the phase of a carrier wave of the first RZ-DQPSK signal by −π/4 in the other optical path of the pair, for transmitting the second RZ-DQPSK signal via the pair of the optical paths;
a second photo detector circuit for converting an optical signal output from said second interferometer into an electrical signal;
a second high-pass filter with a cut-off frequency which is equal or approximately equal to the symbol rate of the RZ-DQPSK optical signal, for filtering the output signal of said second photo detector circuit;
a second BPSK demodulator circuit for demodulating the output signal of said second high-pass filter; and
a signal processing circuit for regenerating data stream from the output of said first and second BPSK demodulator circuits, wherein
said first interferometer outputs a pair of complementary optical signals, and
said first photo detector circuit comprises a pair of photodiodes for respectively converting the pair of the optical signals into a pair of electrical signals and outputs a first differential signal of the pair of electrical signals acquired by the pair of photodiodes, and
said first high-pass filter acquires a first BPSK signal from the first differential signal, and
said first BPSK demodulator circuit demodulates the first BPSK signal, and
said second interferometer outputs a pair of complementary optical signals, and
said second photo detector circuit comprises a pair of photodiodes for respectively converting the pair of the optical signals into a pair of electrical signals and outputs a second differential signal of the pair of electrical signals acquired by the pair of photodiodes, and
said second high-pass filter acquires a second BPSK signal from the second differential signal, and
said second BPSK demodulator circuit demodulates the second BPSK signal.