Interferometric method for preventing mode-hopping in tunable external-cavity semiconductor lasers

A problem in frequency-tuning of external-cavity lasers is mode-hopping between neighbouring external-cavity modes. We demonstrate a new interferometric method for monitoring mode-hoping and an automatic control circuit for a 1.3 ?m grating external-cavity laser that maintains single-mode operation when the lasing frequency is tuned.     

2 cm long monolithic multisection laser for active modelocking at 2.2 GHz

A 2 cm long three-contact multisection laser has been fabricated for modelocking at very low repetition rates. Active modelocking yields 8.0-12.3 ps pulses at 2.2, 4.3 and 6.5 GHz corresponding to the fundamental, first, and second harmonics of the cavity resonance frequency. This device demonstrates the feasibility of fabricating monolithic pulse sources operating at rates as low as the OC-48 Sonet transmission rate (2.488 Gbit/s).<>     

A DBR laser tunable by resistive heating

A distributed Bragg reflector (DBR) laser tuned by resistive heating is presented. It has a tuning range greater than 10 nm with only a 33% reduction in output power and a 10% increase in linewidth. Its behavior is easily modeled, agreeing well with simple theory     

A transimpedance APD optical receiver operating at 10 Gb/s

The authors report an optical receiver which uses a separate-absorption-and-multiplication avalanche photodiode (SAM-APD) to achieve high sensitivity at a bit rate of 10 Gb/s. A transimpedance front end incorporating HEMT devices is used for high bandwidth and low noise. The sensitivity (bit-error rate of 10^-9) is -28.7 dBm for a return-to-zero signal, and -27.0 dBm for a nonreturn-to-zero signal     

Five channel WDM soliton pulse generation using sinusoidally drivenelectroabsorption modulators in 16×1 laser/modulator array

Five wavelength multiplexed pulse streams are generated in an integrated 16×1 laser modulator array by sinusoidally driving five of the potential 16 electroabsorption modulators. At a repetition rate of 2.5 GHz, near transform-limited pulses spaced in wavelength over 7.6 nm are observed. Pseudorandom coding of one channel is achieved by applying an electrical RZ data signal to the modulator     

Image segmentation by a contrario simulation

Segmenting an image into homogeneous regions generally involves a decision criterion to establish whether two adjacent regions are similar. Decisions should be adaptive to get robust and accurate segmentation algorithms, avoid hazardous a priori and have a clear interpretation. We propose a decision process based on a contrario reasoning: two regions are meaningfully different if the probability of observing such a difference in pure noise is very low. Since the existing analytical methods are intractable in our case, we extend them to allow a mixed use of analytical computations and Monte-Carlo simulations. The resulting decision criterion is tested experimentally through a simple merging algorithm, which can be used as a post-filtering and validation step for existing segmentation methods.     

High-temperature quasi-continuous operation of optically pumpedCdZnTe/ZnTe multiple-quantum-well lasers at 620 nm

Low-lasing-threshold quasi-continuous optically pumped II-VI quantum-well lasers were operated well above room temperature at 620 nm. This result shows promise for high-repetition-rate, short-pulse generation by direct modulation of the injection current and also CW operation of future wide-gap II-VI diode lasers above room temperature     

4.5 Gbit/s modelocked extended-cavity laser with a monolithically integrated electroabsorption modulator

An electroabsorption modulator has been monolithically integrated with an extended-cavity laser, which incorporates a Bragg reflector. Actively modelocked at the fundamental cavity resonance frequency of 4.5 GHz, the laser provides a train of 6.3 ps pulses with a centre wavelength of 1544 nm onto which data is encoded by the modulator. This 4.5 Gbit/s single-chip transmitter is suitable for systems employing short optical pulses.<>     

A stripe-geometry double-heterostructure amplified-spontaneous-emission (superluminescent) diode

A superluminescent diode (SLD) which simultaneously generates incoherent optical radiation and amplifies this radiation has been made from double-heterostructure (DH) AlxGa1-xAs-GaAs material previously used to make lasers and light-emitting diodes having superior characteristics. The device configuration was similar to that of the stripe-geometry DH injection laser, except that optical feedback was suppressed by providing absorption for the backward waves in the cavity. A single-pass gain device resulted. The stripe contacts were 12.5 μm or 25 μm in width, and from 0.125 to 1.5 mm in length. The single-pass gain was found to be proportional to (e^{betaL} -1)/beta, where β is a gain coefficient depending on the pump current andLis the stripe length. The structure was found to favor low-order modes, and the half-power radiation beam angle in the junction plane was measured to be6-10deg. The radiation field was slightly TE polarized, and it exhibited a spectral width of 50-85 Å. When operated in the pulsed mode, the longest diodes (1.5 mm) radiated 60-mW peak power at a peak-current density of 6.5 kA/cm²(2.5-A current) and about 10 mW at 3 kA/cm². The external differential quantum efficiency was 3.5 percent. Due to the relatively narrow radiation pattern of the SLD, more than 80 percent of the radiation from this source has been coupled into a multimode optical fiber with a numerical aperture of 0.63. These measured characteristics were found to conform to those calculated on the basis of a simple model.     

A simple interferometric method for monitoring mode hopping intunable external-cavity semiconductor lasers

The mode-hopping events in a 1.3-μm grating-tuned external-cavity laser are analyzed on the basis of interferometric measurements. The average frequency of mode hopping for a 7.5-cm external-cavity laser is estimated and is expressed as f_c =2.7×10⁶×exp [-1.7/(I/I _th-1)] (hertz) for 0.07⩽(I/I_th -1)⩽0.8 (I=injection current and I_th =threshold current). The mode-hopping monitoring signal was negatively fed back to the laser using an automatic control circuit which maintained single-mode operation while the wavelength of the grating external-cavity laser was tuned