Gregory E. Obarski

Transfer Standard for the Spectral Density of Relative Intensity Noise of Optical Fiber Sources Near 1550 nm

We have developed a transfer standard for the spectral density of relative intensity noise (RIN) of optical fiber sources near 1550 nm. Amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA), when it is optically filtered over a narrow band (<5 nm), yields a stable RIN spectrum that is practically constant to several tens of gigahertz. The RIN is calculated from the power spectral density as measured with a calibrated optical spectrum analyzer. For a typical device it is -110 dB/Hz, with uncertainty ⩽0.12 dB/Hz. The invariance of the RIN under attenuation yields a considerable dynamic range with respect to rf noise levels. Results are compared with those from a second method that uses a distributed-feedback laser (DFB) that has a Poisson-limited RIN. Application of each method to the same RIN measurement system yields frequency-dependent calibration functions that, when they are averaged, differ by ⩽0.2 dB.

Precise calibration for optical amplifier noise figure measurement using the RIN subtraction method

Calibration with a relative-intensity noise (RIN) standard significantly improves the RIN subtraction method for measuring the noise figure of an erbium-doped fiber amplifier (EDFA). The typical standard uncertainty in our measurement is /spl les/0.15 dB.

Arrayed DBR waveguide lasers for the ITU grid

We have successfully demonstrated an array of monolithic, single-frequency DBR waveguide lasers that operate near 1536 nm. Single transverse mode waveguides in the 1500 nn telecommunications band were fabricated in a commercially available phosphate glass that was codoped with 1/spl times/10/sup 20/ Er/sup 3+/ ions/cm/sup 3/ and 4/spl times/10/sup 20/ Yb/sup 3+/ ions/cm/sup 3/. Phosphate glass is a very good host compared to silica for erbium ions since the sensitization efficiency is nearly unity and large doping concentrations are possible before the onset of concentration quenching. We have performed waveguide laser simulations correlated to experimental results that indicate the Yb-Er energy transfer efficiency is greater than 95% in this glass. Waveguides are formed by potassium for sodium thermal ion-exchange. We have also used a field assisted process to form waveguides in codoped phosphate glass. Tests of thermal ion exchanged Fabry-Perot lasers without DBR gratings have shown that slope efficiencies of 28% are possible with thresholds as low as 25 mW of coupled 980 nm pump power, Similar results have been achieved using field assisted ion exchanged waveguides. We have also shown these lasers can be tuned from 1525 to 1595 nm.

Surface-emitting laser diode beam characterization (Abstract Only)

We have conducted thorough analysis of a vertical-cavity surface-emitting laser (VCSEL) diode which produces TEM01* (donut) and higher-order modes. Our analysis includes the following quantities as a function of drive current: optical power, spectral content, relative intensity noise (RIN) up to 100 MHz, and beam characterization parameters. While this VCSEL produces higher-order modes which are not affected by optical feedback, its optical power (0.05 mW for TEM01*), long term stability, and sensitivity to collimating lens position make it a doubtful candidate for use in a beam characterization round robin. Also, we hope to present recently acquired data from the diode-pumped tunable transverse mode laser developed in Berlin and tested at NIST.

Multigigahertz relative intensity noise of an InGaAsP laser at cryogenic temperature

Using a well-characterized measurement system having 22 GHz bandwidth, we show that the relative intensity noise (RIN) of a commercial InGaAsP laser (multi-longitudinal mode) operating at 77 K is significantly reduced below room temperature values over a broad frequency range. For laser operation with equal drive currents at both temperatures, a RIN reduction of as high as approximately equal to 24 dB/Hz is observed in the frequency region of relaxation oscillations, while for operation with equal power outputs the maximum reduction observed is approximately equal to 14 dB/Hz.

Relative Intensity Noise of an InGaAsP Laser over a 22 GHz Bandwidth at Cryogenic and Room Temperature

Cryogenic operation of semiconductor lasers offers the potential for greater bandwidth due to improved frequency modulation response,1,2 lower threshold current, and higher power efficiency3 over that of room temperature operation. The 3- dB electrical modulation bandwidth can be determined from the relaxation oscillation frequency as located from the peak in the relative intensity noise (RIN) spectrum.4 In this report we show that the cryogenic RIN of a commercial 1.5 μm InGaAsP laser is significantly reduced below room temperature values over a major portion of a 22 GHz measurement bandwidth, yielding increased potential for ultrahigh speed modulation. Low RIN lasers are also useful for determining the optimum performance of advanced photodetectors, for measuring noise figure of optical fiber amplifiers, and as reference standards for the comparison and evaluation of RIN measurements.

Scanning confocal microscopy for measuring diameter and linewidth: numerical modeling

The image of a circular edge was calculated as determined by a scanning confocal microscope with fully coherent illumination. In scalar theory, the quarter-intensity point locates the geometrical-optics image of a straight edge. For a circular object, however, the quarter- intensity point is displaced from the geometrical-optics image of the edge according to the diameter of the object. For example, for an object that has a diameter of 21 resolution limits the displacement error is approximately 0.01 resolution limits. The error that results from locating the quarter-intensity point for diameters as small as 1 resolution limit is given. The error is even greater if the object is scanned off-axis. For example, the error for an object whose diameter is 21 resolution limits and which is scanned 3 resolution limits off-axis is approximately 0.45 resolution limits. Finally errors are calculated for vertical lines of width as small as 1 resolution limit.

Lambdameter for accurate stability measurements of optical transmitters

To accurately measure wavelengths of 1.3 and 1.5 micrometers single-mode sources, we developed a lambdameter that can be used in the near IR and the red regions of the spectrum. Wavelength accuracy and resolution are approximately equals 0.1 ppm (parts per million) at 0.633 micrometers . They were measured by comparing each of two adjacent modes of a HeNe laser, frequency- stabilized by a polarization technique, with a single mode from a second frequency-stabilized HeNe laser. We also verified the wavelength of the reference laser with an accuracy of 1 ppm by comparing it with the 1.52 micrometers HeNe laser line. The uncertainty in wavelength of the 1.52 micrometers HeNe laser is limited to the width of the Doppler gain curve, whose peak is known within 0.2 ppm. We describe our lambdameter and the performance of its reference laser as a wavelength transfer standard. Measurements on a commercially packaged 1.52 micrometers distributed-feedback (DBF) laser diode transmitter show that its wavelength fluctuates by at least 1 ppm during normal changes in room temperature.