Optical Signal Source Research Articles

Wavelength selection strategies to enhance referencing in LED based optical sensors

LED emission spectra vary significantly with the operating temperature of the device and therefore most intensity-modulated optical fibre sensors employing LEDs as the optical signal source suffer from the associated measurement errors. Although, the advanced types of intensity-modulated sensors will often incorporate thermal stabilisation or a mechanism for reducing these errors, such implementations often involve considerable amount of increased system complexity. An alternative strategy called wavelength thermal matching (WTM) is described which depends on a careful selection of the signal channel centre-wavelengths to provide freedom from the errors caused by the LED thermal instability. Two distinct methods of minimising the effect of LED thermal instability are identified to achieve either thermally stabilised signals or thermally compensated signals. Thermally stabilised optical signals can be used to improve the performance of many single channel sensor systems as well as to simplify the design of stable LED signal sources employed for test and measurement applications. The strategy of providing thermally compensated signals has immediate implications for the dual wavelength referencing schemes used in intensity-modulated optical fibre sensors which can be enhanced in their effectiveness by selecting the signal wavelength bands so that differential variations between the signals due to the LED thermal effects are minimised.

Intermodulation distortion in 48 TV channel FM-FDM optical transmission

The effect of dispersive fibers on multichannel FM video transmission characteristics is experimentally investigated. A directly modulated 1.5 mu m distributed feedback laser diode (DFB-LD) and a LiNbO/sub 3/ external modulator with a continuous-wave (CW) 1.55 mu m DFB-LD are used as optical signal sources. When the 1.3 mu m single-mode fiber (SMF) is used, directly modulated DFB-LDs suffer from second-and third-order intermodulation distortion (IMD). The external modulator realizes good transmission characteristics irrespective of the fiber's dispersion value. This means that external modulators can realize multichannel FM television signal transmission systems that use the standard 1.3 mu m SMF already installed in fiber optic subscriber loops.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Baseband frequency response measurement system for optical components

Knowledge of the baseband frequency responses of optical components is prerequisite to the design of optical fiber transmission systems. The sweep-frequency method is effective for obtaining frequency responses because of its large SNR. However, the use of GaAs lasers as optical signal sources involves several problems such as resonances below 1 GHz and spectrum broadening. In order to overcome these problems, a single transverse mode GaAs laser called a buried heterostructure GaAs laser was employed as an optical signal source in the sweep-frequency measurement system. This measurement system has a wideband flat sweep-frequency range of 0.5-1300 MHz as well as a wide dynamic range of more than 60 dB at optical levels.

640 Mbit/s Pulse-Code Modulation with a Ga/sub 1-x/ Al/sub x/As: GaAs LED

In this correspondence we report pulse-code modulation (PCM) of an LED at 640 Mbit/s. This LED was especially designed as a fast optical signal source for optical fiber transmission and its design and performance parameters are described.

640 Mbit/s pulse-code modulation with a Ga1-xAlxAs: GaAs LED

In this correspondence we report pulse-code modulation (PCM) of an LED at 640 Mbit/s. This LED was especially designed as a fast optical signal source for optical fiber transmission and its design and performance parameters are described.

Optimal wind velocity estimation

A new approach to remote wind velocity sensing is proposed, developed, and tested. Wind estimates were obtained from the spectral signature of an optical signal source. The signal was derived from the random scattering of a laser beam in the lower atmosphere. A quasi-supervised learning machine, using a minimal variance linear estimator, was used to produce the wind velocity estimate. New stopping rules, used to guarantee the convergence of the learning process, were also developed. Field experimentation indicates that the new system will perform well, even under extreme optical nonlinear conditions due to severe turbulence.

Measurement of nonlinear distortion in photodiodes

The 2nd-order nonlinear distortion of avalanche photodiodes (TIXL-55, C-20817) was found to be less than −60 dB for an incident average optical power of −20 dBm modulated at a peak depth of 75%. p-i-n photodiodes (HP 5082-4207) were found to have 2nd-order-distortion levels below the noise level. A method using three independent optical signal sources was employed in the measurement.