Steven T. Johns

Gamma-ray induced responses in an erbium doped fiber laser

Many people are investigating photonic analog-to-digital converters (ADCs) for use in a digital receiver as a plausible solution to increase the bandwidth and resolution over that currently offered by electronic ADCs. A key component of a photonic ADC is a mode-locked fiber laser. A preliminary evaluation of the capability of utilizing this type of laser in space based ionizing environments and applications is required. This paper explains the effects of exposing an erbium-doped fiber laser (EDFL) to a total gamma-ray dose of 1 Mrad (Si). The performance of the laser is characterized in a passive fashion, i.e. before and after the irradiation. Predictions are offered to the extent of radiation induced damage that the fiber laser can endure before breaking down. The results of the evaluation will allow for further optimization of the EDFL for use in space-based architectures and applications.

Comparison of radiation-induced passive and dynamic responses in two erbium-doped fiber lasers

Erbium-doped fiber lasers (EDFLs) may soon find applications in space as high bit rate optical communication systems and photonic analog-to-digital converters (ADCs). The rapid advancement in digital signal processing systems has led to an increased interest in the direct digitization of high-frequency analog signals. The potential high bandwidth, reduced weight, and reduced power requirements makes photonics an attractive technology for wide-band signal conversion as well as for use in space-based platforms. It is anticipated that photonic ADCs will be able to operate at sampling rates and resolutions far greater than current electronic ADCs. The high repetition rates and narrow pulse widths produced by EDFLs allow for high-speed impulse sampling of analog signals thus making it a vital component of a photonic ADC. In this paper we compare the results of exposing two differently constructed erbium-doped mode-locked fiber lasers (EDFLs) to gamma-rays. Each experiment is fully explained. The performance of EDFL1 was characterized in a passive fashion, i.e. before and after the irradiation. EDFL2s performance was monitored in situ. The onset, growth and extent of ionization induced damage under time-resolved operational conditions is presented. The in situ studies clearly revealed ionization induced shifts of the EDFL optical spectrum to shorter wavelengths and with corresponding changes to the emission spectral width. The results of the evaluation will allow for further optimization of an EDFL for use in space-based architectures and applications.

Time-division optical micro-area networks

Optical micro-area networks (iANs) are proposed as a way of providing flexible communications among VLSI processors and eliminate electrical I/O bottlenecks. Sharedmedium multiple access protocols in jtANs can avoid the access delays associated with statistical multiple access protocols (which are unacceptable in multiprocessor applications) and increase the throughput at the expense of wasting optical bandwidth. Time-division multiple access (TDMA) may be more practical to implement in a pAN than other shared-medium multiple access protocols such as frequency-division or code-division. Since the total throughput of TDMA is given by the inverse of the optical pulsewidth the throughput can be increased by making the pulse width small. Accomplishing this goal requires avoiding the use of low-bandwidth electronics in the portion of the iAN that directly processes these short pulses. Instead optical processing can be used in those protions of the network. The architecture of a TDMA pAN which uses optical multiple access processing and is self-clocking is described in detail. Experimental demonstrations of key subsytems for optically generating modulating synchronizing delaying and correlating short optical pulses are presented. The feasibility of a variable-integer-delay line which provides rapid tuing wide tuning range and high precision is demonstrated. A transmitter consisting of a mode-locked laser with an external modulator is used in the TDMA iAN since arbitrarily short pulses can be controlled with a modulator that need only operate at the bit rate which translates© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Gamma-ray-induced damage and recovery behavior in an erbium-doped fiber laser

Erbium-doped fiber lasers (EDFLs) may soon find applications in space as high bit rate optical communication systems and photonic analog-to-digital converters (ADCs). The rapid advancement in digital signal processing systems has led to an increased interest in the direct digitization of high- frequency analog signals. The potential high bandwidth, reduced weight, and reduced power requirements makes photonics an attractive technology for wide-band signal conversion as well as for use in space-based platforms. It is anticipated that photonic ADCs will be able to operate at sampling rates and resolutions far greater than current electronic ADCs. The high repetition rates and narrow pulse widths produced by EDFLs allow for high-speed impulse sampling of analog signals thus making it a vital component of a photonic ADC. In this paper we report on the in situ gamma-ray irradiation of an actively mode-locked EDFL operating at 1530 nm. The onset, growth and extent of ionization induced damage under time-resolved operational conditions is presented. The laser consisted of approximately 3 meters of erbium-doped fiber pumped by a laser diode operating at 980 nm. The picosecond pulses produced by the laser were initiated and controlled by a Mach-Zehnder lithium niobate electro-optic modulator. The active mode-locking element allowed for the precise timing control of the laser repetition rate which is critical in high-speed optical networking systems as well as in photonic ADCs.

Harmonically mode-locked laser and applications

We have constructed a harmonically modelocked laser that includes an electronically driven modulator and an intracavity Fabry-Perot etalon. We use experimentally observed performance of this laser, and number simulations based on the operating parameters of this laser, to examine strategies for generating stable synchronized trains of ultrashort duration solitons at multi-GHz repetition rates. Introduction of a saturable absorber based on a mechanism that both saturates and recovers promptly is examined. This strategy provides means of generating stable trains of solitons, where the soliton durations are of the order of a few ps or less, as well as synchronizing those trains with optical pulsewidth precision. We identify a rapidly saturating and rapidly recovering saturable absorber with a shorter pathlength as a potentially useful improvement on the nonlinear loop mirror. Significant work remains, but generation and distribution of these synchronized ultrashort duration soliton trains over networks on a scale of km or more appear feasible.

Free-space optical TDM switch

A free space optical TDM switch at 1.32 micron is experimentally studied. The architecture of a TDM circuit switching system with a fixed transmitter and tunable receiver assignment is described. Since each user of the TDM switching system is assigned a time slot on the time frame, the corresponding receiver at the output looks only in the preassigned time slot for signal recovery. The electrical data signal from the input source is used to gate the optical pulses from the centralized source for the duration of the signal. It is concluded that large input/output switching systems are feasible.

Harmonically mode-locked erbium-doped waveguide laser

The generation of ultrastable picosecond pulses in the 1550 nm range is required for numerous applications that include photonic analog-to-digital converter systems and high-bit rate optical communication systems. Mode-locked erbium-doped fiber ring lasers (EDFLs) are typically used to generate pulses at this wavelength. In addition to timing stability and output power, the physical size of the laser cavity is of primary importance to the Air Force. The length of the erbium (Er)-doped fiber used as the gain medium may be on the order of meters or even tens of meters which adds complexity to packaging. However, with the recent advancements in the production of multi-component glasses, higher doping concentrations can be achieved as compared to silicate glasses. Even more recent is the introduction of Er-doped multi-component glass waveguides, thus allowing the overall footprint of the gain medium to be reduced. We have constructed a novel harmonically mode-locked fiber ring laser using the Er-doped multi-component glass waveguide as the gain medium. The performance characteristics of this Er-doped waveguide laser (EDWL) including pulse width, spectral width, harmonic suppression, optical output power, laser stability and single sideband residual phase noise will be discussed in this paper.

Performance characteristics of a mode-locked erbium-doped fiber ring laser as a function of erbium ion concentration

The generation of ultrastable picosecond pulses in the 1550 nm range is required for numerous applications including photonic analog-to-digital converter systems and high-bit rate optical communication systems. Mode-locked erbium-doped fiber ring lasers are typically used to generate pulses at this wavelength. In addition to stability and output power, the physical size of the laser cavity is of primary importance. The length of the erbium-doped fiber used as the gain medium may be on the order of meters or even tens of meters which makes packing of the laser rather difficult. However the length of the gain medium can often be reduced if the erbium ion concentration within the fiber is increased. This paper will investigate the performance of an erbium-doped fiber ring laser as a function of ion concentration within the gain medium. Results will be presented for mode-locked lasers consisting of Lucent HE980, HG980 and HC erbium-doped fibers. The parameters that will be compared between the lasers include the output power as a function of length and concentration, optical pulse width and spectral bandwidth. Phase noise measurements of the laser output will also be presented.

Development of semiconductor saturable absorbers for use in photonic analog-to-digital converters

Wide bandwidth and high-resolution analog to digital converters (ADCs) are required for the next generation of sensor systems. Progress at advancing the electronic ADC modules has been very slow due in large part to the difficulties in fabricating the electronic circuitry required for very high resolution and high sampling rate converters. It is anticipated that the use of photonic ADCs will far surpass the performance of electronic ADCs in terms of both sampling speed and resolution. We have recently designed a novel photonic ADC module that incorporates the use of semiconductor saturable absorbers to perform the data quantization at speeds in the tens of GHz regime. Experimental material characterization results including the nonlinear transmission and the recovery time of the semiconductor saturable absorbers use din the data conversion process will be presented. Different material parameters will also be analyzed including the effects of low temperature growth, band-edge position, and strain on these material properties.

Saturable absorber mode-locked femtosecond Cr4:YAG laser

We have demonstrated self-starting passive mode-locking of a chromium-doped:YAG (Cr4+:YAG) laser using a saturable absorber mirror (SAM) structure. Highly stable femtosecond pulses tunable from 1488 to 1535 nm were generated. Average TEM00 output powers ranged from 40 to 80 mW with a minimum pulse width of 120 fs measured at 1488 nm. The generation of ultrashort pulses in solid-state lasers using the Kerr lens modelocking (KLM) technique has been the center of much attention in recent years. Sub 100 fs pulses have been produced using many different laser systems. However, the KLM process is very sensitive to cavity alignment and is easily perturbed by mechanical vibrations and pump power fluctuations. A more reliable process makes use of a saturable absorber to start and stabilize the soliton formation process. The saturable absorber eliminates the need for critical cavity alignment. The femtosecond pulse train produced by the SAM structure in the Cr4+:YAG laser system is highly stable over very long periods of time. The wide tunability of the Cr4+:YAG laser throughout the 1.5 micrometer transmission window of optical fiber makes it an ideal spectroscopic source for the characterization and development of novel materials and devices for ultrafast optical interconnects.