Synchronisation method of a local oscillator in an optical homodyne receiver

The present invention is concerned with highly sensitive receivers for free space laser communication systems. Such systems should enable broadband communication over large distances, for example between satellites or between a ground station and a satellite, in the near future.

The invention relates to a method for synchronizing a local oscillator in a homodyne receiver with respect to an optical communication signal, the homodyne receiver not only containing the local oscillator with an adjustable instantaneous frequency, but also a mixer, a power detector and control electronics.

In order to achieve the best possible sensitivity (i.e. the smallest input signal power required for a given bit error probability) in an optical receiver, binary phase shift keying is chosen as the modulation format and homodyne reception as the receiver concept. The theoretically achievable sensitivity of such a receiver is 9 photons per bit for a bit error probability of 10 '⁹. In order to come as close as possible to this theoretical limit, the local oscillator (LO) of the receiver must be phase-locked synchronized with the carrier of the data signal. This is generally accomplished by a phase locked loop which affects the instantaneous frequency (and therefore also the phase) of the local oscillator. The aim is to achieve a constant phase difference between the data carrier and the local oscillator in the optical mixer element of the receiver. Only then does the receiver work as a homodyne receiver, ie the optical, phase-modulated input signal is converted directly into the (electrical) baseband.

The synchronization of the local oscillator is one of the main problems in the practical implementation of the homodyne receiver concept. The most important requirements are:

1. Deviations from the state of perfect synchronization must be kept as small as possible.

2. The portion of the optical input power that is necessary to determine the phase error should also be as small as possible.

3. The components for determining and minimizing the phase error must be reliable and must not (or only insignificantly) attenuate the data signal.

State of the art

A number of synchronization methods have been proposed or verified experimentally so far. The concepts listed below mainly differ in the way in which the phase error is determined:

1. With the pilot carrier concept [1], a small residual carrier is superimposed on the data signal. This method requires a DC coupled photodetector.

2. The so-called Costas loop [2] does not require a direct current coupling, but an optical 90 ° hybrid and two broadband receiver branches.

3. The syncbit concept [3, 4] avoids the problems of the above-mentioned methods by inserting phase-shifted synchronization bits into the data stream. This concept requires complex, fast digital electronics to track down the synchronization bits and determine the phase error. The Syncbit concept was developed by B. Wanderoth in the article 1064nm, 565Mbit / s PSK transmission experiment with homodyne receiver using synchronization bits, Electron. Lett., 27 (19), pp. 1692-1693, 1991 and in the German patent with the title “Method for Carrier Recovery in an Optical PSK Homodyne Receiver”, DE 41 10 138-C1, 1991. The sync bit concept uses phase-shifted synchronization bits. This concept requires complex, fast digital electronics to track down the synchronization bits and determine phase errors.

4th With the switched carrier method [5], the phase of the carrier signal is slightly modulated in the transmitter. In the receiver, the phase error is determined by multiplying the baseband signal with the regenerated data signal.

5. A similar concept [6] varies the phase deviation in the transmitter and can reduce the phase error

AT 410 153 B by envelope detection - without the aid of the data regenerator - determine.

6th Another novel concept [7] exclusively uses the usually existing limitation of the bandwidth of the phase modulator in the transmitter and the resulting asymmetry of the eye diagram, but requires the recovered symbol clock to determine the phase error.

US Patent 3,746,998 describes a system for the automatic detection of a carrier signal. The system described is designed for use in a Costas phase-locked loop (PLL) receiver. European patent application EP 0 594 352 describes a digital control loop which minimizes peak performance in a multicarrier communication system. The proposed controller derives the control signals for several phase shifters from a single measured variable (peak power) by adjusting each individual phase shifter in one direction in time multiplex and evaluating the influence on the peak power.

Description of the invention

The present invention is a novel method for synchronizing the local oscillator in a homodyne receiver. According to the invention, the following steps are carried out:

- mixing the optical communication signal with an output signal of the local oscillator (i),

- controlling the power detector (e) with an output signal (u (t)) provided at the output of the mixer (a),

- evaluate the power of the output signal (u (t)) of the mixer (a) by the power detector (e),

- regulate the instantaneous frequency of the local oscillator (i) in order to maximize the output signal of the power detector (e).

One possible implementation, as it would be useful for use in the optical field, is shown in Fig. 1. The invention is based on the fact that the power of the baseband signal at the output of the mixer element (a) can be used as an indicator of the phase difference between the carrier of the data signal and the local oscillator. It is clear that this power is maximum when the desired phase state is reached, and minimum when the phase difference deviates from the optimum by ± 90 °. The desired phase state can thus be acquired and maintained with a maximum regulator.

Mathematical description

The input signals of the 180 ° hybrid (b) are fed through the phasors e_s(t) = E_se^{j (}^^{+ d (t))} or β | = Ε | β^ΚΦ,) (1). E._s and £ 7 denote the amplitudes of the electric fields, φ_ε and φ, describe the current phase of the unmodulated data signal or of the local oscillator. d (t) is the stochastic modulation signal, which varies between ± π / 2. It can be shown that the output of the electrical power sensor (e), u²(t), the following equation is sufficient:

cos2 (^_s -φ,), where (2) = [z (t) * cos d (t)]^z , (3) and ß = [z (t) * sin d (t) f

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(5)

AT 410 153 Β applies. The bar means averaging over a large number of bits, the asterisk means convolution. S.₇and S.₂ denote the sensitivity of the photodiodes (c) and z (t) is the impulse response of the AC coupled amplifier (d) (if a separate filter is used, its transfer function is included in z (t)). Equation (2) means that the output of the power detector depends only on the phase difference between the data signal and the local oscillator (0_S. - φι) depends and between al₀² (Minimum) and ßl₀² (Maximum) varies.

To get the desired state - φι = ± π / 2, u must²(t) can be maximized. A very suitable method for this purpose is to change the phase of the local oscillator (^_s) to apply a small sinusoidal disturbance (h) and the resulting power fluctuation u²(t) to demodulate synchronously (f) (the frequency of the interference should be well above the bandwidth of the control loop). This measure results in a sinusoidal phase detector characteristic at the input of the loop filter (g). The output of the loop filter controls the instantaneous frequency of the local oscillator, which closes the phase-locked loop.

Advantages compared to previous methods • Compared to the pilot carrier concept, the amplifier (d) in the invented method can be AC-coupled.

• Compared to the Costas loop, if the method is implemented in the optical field, only a 180 ° hybrid is required, which is nowadays easy to manufacture in the form of a symmetrical directional coupler.

• Compared to the other, recently developed methods [3, 4, 5, 6, 7]

- the invented phase-locked loop works completely independently of symbol clock and data regeneration;

- With the exception of a single power detector, the invented method does not require any broadband circuits,

- the invented method does not require any modification of the transmitter.

literature

[1] AL Scholtz et al. Realization of a, 10pm homodyne receiver. J. Lightwave Technol., LT-5 (4): 625-632. 1987.

[2] LG Kazovsky. Decision-driven phase-locked loop for Optical homodyne receivers: Performance analysis and laser line width requirements. J. Lightwave Technol., LT-3 (6): 12381247, 1985.

[3] B. Wanderoth. 1064nm, 565Mbit / s PSK transmission experiment with homodyne receiver using synchronization bits. Electron. Lett., 27 (19): 1692-1693, 1991.

[4] B. Wanderoth. Method for carrier recovery in a PSK homodyne optical receiver. Patent DE 41 10 138 C1, 1991.

[5] W. Glatt et al. Optical PSK homodyne system using a switched residual carrier for phase synchronization. Electron. Lett., 32 (15): 1386-1387,1996.

[6] C. Rapp. Modulated residual carrier method with envelope processing. In: Proc. CRL International Symposium on Optical Communications and Sensing towards the next Century, Tokyo, pp. 215-216, 1999.

[7] F. David. New phase synchronization method for optical BPSK homodyne receivers. In: Proc. CLEO'99, page 434, 1999.