Walter R. Leeb

Electrooptic gigahertz light-pipe modulator for CO 2 laser wavelength

We have built a traveling-wave electrooptic modulator for 10.6 μm where the optical beam propagates in a highly oversized waveguide. Three CdTe-crystals measuring 35 \times 0.8 \times 0.8 mm3each are cascaded to yield an aspect ratio of 131. Velocity matching between optical and modulating waves and a line impedance of 100 Ω are achieved by proper choice of dielectric embedding and of electrode width. Measured characteristics are: optical transmission 96 percent, maximum input VSWR 1.25:1, half-wave voltage 450 V, and frequency response ±1.1 dB over a 0.006-1 GHz range.

Experimental determination of power penalty contributions in an optical Costas-type phase-locked loop receiver

We designed, realized and tested a 140 Mbit/s P5K optical homodyne receiver using diode-pumped Nd:YAG ring lasers at X = 1.064im. Phase synchronization between the input signal and the local oscillator laser was achieved using a Costas-type phase-locked ioop. Pre-processing of the optical signals to be synchronized was done with a basically lossless six-port 90° hybrid. Each one of the employed front ends was implemented by means of a transimpedance preamplifier and by two InGaAs photodiodes operating in a balanced manner. We report on the receiver configuration, emphasizing the realization of the optical pre-processing unit and the front ends. Furthermore, we present measurement results characterizing the receiver performance by the bit-error rate dependence on optical input signal power. The total degradation of the receiver sensitivity -as compared to the ideal case of shot-noise limitation - was 5.9dB.

Experimental results on an optical array antenna for non-mechanical beam steering

A beam-steering experiment was carried out to demonstrate the basic behavior of an optical array antenna (OAA) whose subantenna fields are phased, permitting nonmechanical steering of the outgoing laser beam in analogy to the microwave regime. Good correspondence was obtained between the measured far-field intensity patterns and the calculated patterns. The new OAA concept is sufficiently accurate, is quite insensitive against disturbances, and consumes negligible optical power. Steering angles within a range of 1 mrad were obtained. The settling time of a commanded steering angle is about 0.7 ms; it is mainly determined by the response of the phase actuators employed.

Heterodyne acquisition and tracking in a free-space diode laser link

A laboratory model of an optical intersatellite link employing InGaAs DFB semiconductor lasers operating at a wavelength of 1.55 micron was designed and realized. Heterodyne sensing was used for both the spatial acquisition and the spatial tracking processes. The function of a quadrant detector was realized by splitting the superimposed beam at the top of a reflecting pyramid into four subbeams. The angular resolution achieved - without using a telescope - is less than 5 microrad at a detector field of view of 1 mrad. The transmitter laser can be moved within a transverse plane along circular tracks. A microcomputer controls the receiver operation. During the acquisition process spiral scanning of the area of uncertainty is performed. For each search position the local oscillator laser is swept until a beat signal at 700 MHz is detected. Acquisition times of typically less than 16 s for a 200-element uncertainty area and tracking accuracies better than +/- 50 microrad for any examined test condition were achieved.

Single-mode laser frequency modulation

This paper presents an analysis of the frequency response of single-mode laser frequency modulation. The method employed is based on the combined concepts of an active Fabry-Perot interferometer and of internal phase modulation, and is very transparent. The calculation of the sideband amplitudes and phases of the laser output reveals two main regions of the frequency response: for modulating frequencies lower than the cavity mode spacing the laser generates an ideally frequency-modulated signal. At higher frequencies the sideband amplitude versus modulating frequency shows an oscillatory behavior. Here, the response is mainly determined by the cavity mode spacing frequency and by the position of the modulator within the cavity. Comparing the theoretical results with previously reported experiments, very good agreement was found.

Single mode CO 2 laser frequency modulation up to 350 MHz

Experiments on internal frequency modulation (FM) of a CO 2 laser showed no limitation of FM by the linewidth. However, distortions in the form of strong enhancement of sideband amplitude arise for frequencies equal to the cavity resonant frequencies, most pronounced if the modulator is positioned near a cavity mirror.

Optical phased array antennas for free space laser communications

The principle of phased array microwave antennas can be applied at optical frequencies. The far-field antenna pattern is found by spatial Fourier transform of the optical field distribution across the subaperture plane. Inertia-free antenna pattern steering can be accomplished by proper phasing of the subaperture waves. A tolerance analysis shows that the required accuracy of phase relationship and subantenna alignment can be obtained in practice when implementing control loops. We develop schemes for both transmit and receive array antennas. Experiments carried out at (lambda) equals 1.06 micrometers demonstrated both modes of operation. Optical array antennas may be applied advantageously for fine pointing in intersatellite data links and in space lidar systems.

Homodyne Receiver Concepts For CO 2 Laser Intersatellite Links

After discussing the sensitivity limit of optical PSK homodyne receivers we describe the realisation and properties of a mechanically cooled photodiode/preamplifier module used in our breadboard model of a 10 pm receiver. Several receiver concepts are described which have in common frequency and phase synchronization between input signal and local laser oscillator signal with a phase-locked loop (PLL) control circuit. Means to avoid DC-coupling of the PLL are discussed. The results of bit error measurements with an ordinary, DC-coupled homodyne receiver operated at 140 Mbit/s are presented.

Adaptive optical multiaperture receive antenna for coherent intersatellite communications

The concept of an adaptive receive telescope array (RTA) for coherent optical space communications is presented. The RTA consists of N equals 2K, e.g. 16, subtelescopes, N polarization-maintaining single-mode fibers, N optical phase actuators, a binary tree of N - 1 symmetrical polarization-maintaining directional couplers, N - 1 optical power sensors, and a digital control unit. The output interface, a polarization-maintaining single-mode fiber, can be efficiently coupled to a subsequent coherent receiver. Within a subtelescopes field-of-view, the control unit adapts the subtelescope phases (pistons) to the direction of the incident wavefront, thus maximizing the strength of the optical output field. The RTA is transparent, i.e. it operates independently of the modulation format employed. The feasibility of the RTA concept was demonstrated in a laboratory experiment. The implemented four-aperture antenna operates at a wavelength of 1064 nm. At an optical power level of 1 nW per subaperture, the experimental system combines the optical input signals with an efficiency greater than 99%. A step-shaped change of input wavefront direction is automatically compensated within 1 ms.

Highly sensitive 565-Mbit/s optical heterodyne receiver demonstrator for λ=1.064 μm with automatic frequency acquisition

We have designed, realized and tested a coherent optical 565 Mbit/s PSK heterodyne receiver demonstrator for space communications which employs a diode-pumped Nd:YAG ring laser at (lambda) equals 1064 micrometers as local oscillator (LO). The entire system was built as a robust and compact unit, with a fiber connector input for the received signal. The required optical input power is 22 photons/bit for a bit error probability of 10-6, corresponding to a sensitivity degradation of 3 dB compared to an ideal, shot noise limited PSK heterodyne system. Reliable, fully automatic frequency acquisition is achieved employing a microcomputer controlled two-speed algorithm. For initial frequency differences of up to 30 GHz the average acquisition time is 140 s. A frequency tracking subunit controls the LO laser frequency to keep the intermediate frequency constant, even in the presence of Doppler shift. Doppler shift rates of up to 11.5 MHz/s can be compensated.