Asif A. Godil

TIME-TO-FREQUENCY CONVERTER FOR MEASURING PICOSECOND OPTICAL PULSES

We present a new technique for measuring the intensity I(t) of optical pulses using a temporal optical system. A diffraction grating pair followed by a microwave‐driven, optical phase modulator configured as a time lens is used to uniquely map the pulse shape from the time domain to the frequency domain, allowing measurement of the pulse shape with a spectrometer. We discuss the theory of operation and present experimental results illustrating 3 ps time resolution.

Acousto-optic modulators for optical fibers using Hertzian contact with a grooved transducer substrate

The authors describe acousto-optic modulators for optical fibers, fabricated by the deposition of thin zinc-oxide transducers onto specially lapped glass capillary tubes. By exposing the inner surface of the capillary and contacting it to an optical fiber of the same diameter, a good Hertzian contact can be easily attained, providing efficient throughput of acoustic energy at frequencies in excess of 1 GHz. A phase modulator for a single-mode fiber has been fabricated in such a manner, achieving a maximum phase retardation of 1.0 rad/W input electrical power at 416 MHz. Its frequency, power, and polarization characteristics are discussed. Also presented are data from a frequency shifter/modal coupler for a two-mode optical fiber. This device achieved 6% power coupling from the fundamental optical mode to the second-order mode with 500 mW input power at 1.03 GHz. Single sideband modulation was observed with >19-dB suppression of the unwanted sideband. The authors also describe the design of an optical tap using the same technology.<<ETX>>

Time-lens producing 1.9 ps optical pulses

A resonant microwave optical phase modulator in LiNbO3 with multiple passes, based on an off‐axis path in a stable optical resonator, is demonstrated as a time‐lens. With 1 W of cw microwave power at 5.2 GHz, 45 ps pulses at 1.06 μm were temporally focused to 6.7 ps (FWHM). Increasing the drive power to 13 W, at 10% duty cycle, produced 1.9 ps pulses. The aperture of the time‐lens is about 31 ps. This is the first demonstration of a useful time‐lens.

Picosecond time-lenses

There is an interesting analogy between the spatial problem of Fresnel diffraction and the temporal problem of first-order dispersion. This space-time analogy was recently extended to propose a time-domain analog to spatial imaging that allows for the distortionless expansion of compression of optical waveforms in time. This process is called temporal imaging. The extension includes the idea of a time-lens as a dual of a spatial lens (regular lens). The time-lens is simply a quadratic optical phase modulator in time, which is approximated by a portion of a sinusoidal phase modulator. Thus, by using phase modulators as lenses and grating pairs as dispersive elements, complete temporal imaging systems can be constructed in exact duality with spatial imaging systems. However, for practically useful time-lenses, considerable modulation is required at fairly high frequencies. The main body of the paper is the detailed design and development of a practical time-lens. This is addressed in Section II, where a resonant microwave modulator is developed based on a LiNbO/sub 3/ loaded waveguide. Multiple passes are obtained through the modulator using an off-axis path in a stable optical resonator. At 5.2-GHz operation, 44 radians of phase modulation is obtained at 1.06-/spl mu/m wavelength for 13 W of microwave power. This corresponds to a time-lens with 31-ps aperture and 1.9-ps resolution. This was confirmed by demonstrating temporal focusing of 45-ps pulses to 1.9 ps. By optimizing the design of the time-lens and better thermal engineering, it may be possible to obtain subpicosecond resolution. >

Harmonic mode locking of a Nd:BEL laser using a 20-GHz dielectric resonator/optical modulator

A 20-GHz dielectric-resonator/optical modulator is developed and used as an FM mode locker at the 84th harmonic of a conventional 238-MHz diode-pumped Nd:BEL laser cavity. Depending on the mode-locker drive frequency, two distinct regimes of mode locking were observed: 2.9-ps pulses at a repetition rate of 238 MHz and 3.9-ps pulses at a repetition rate of 20 GHz. These are to our knowledge the shortest pulses ever reported for active mode locking of a Nd laser.

All-fiber acoustooptic phase modulators using zinc oxide films on glass fiber

Experiments with an acoustooptic phase modulator for single-mode optical fibers are described. The device, which operates in the 500-MHz frequency range, uses a zinc oxide transducer sputter-deposited on top of a gold film evaporated on the fiber. The device is heat cooled and partially acoustically terminated by gallium, which is also used as the top electrode of the transducer. The maximum modulation observed on a 2-mm length of fiber is 2 rad with an input power of 2 W. >

Detachable 400-MHz acousto-optic phase modulator for a single-mode optical fiber.

A single-mode-fiber phase modulator was constructed by contacting the fiber with a lapped glass capillary tube. The capillarys inner surface provides a long, effectively semicircular contact region to the fiber, allowing throughput of acoustic waves launched from a thin-film ZnO transducer fabricated directly onto the capillarys other lapped face. The device operated at a center frequency of 416 MHz with a FWHM bandwidth of 14 MHz. The maximum phase shift was 0.033 rad/ radicalmw, with a largest measured value of 1.2 rad at 1.3-W input electrical power.

Pulsed FM mode locking of a Nd:BEL laser

A novel but simple and practical mode locker was built and demonstrated for a diode-pumped Nd:BEL laser. Fast electrical pulses from a comb generator drive a LiNbO(3) crystal, which produces pulsed electro-optic phase modulation in the laser cavity. Stable mode-locked pulses of 7.5-ps duration were obtained at a repetition rate of 250 MHz.

Fiber-optic magnetic-force phase modulator

A fiber-optic magnetic-force phase modulator using an aluminium-jacketed current-carrying fiber coil placed in a uniform external magnetic field is presented. The modulator has a center frequency of 30 kHz with a modulation efficiency of 2.4 rad/mA and a Q of 4.6. A theoretical model is developed for the modulator, and its predictions are compared with experimental results. An accurate and simple method of characterizing the phase modulator response is described. >