High-Speed Room-Temperature CO2 Laser Photodetectors
Ten years have passed since the uncooled high-speed HgCdTe C02
laser radiation detectors have been introduced. They are cheap, rugged and easy
to use. In terms of responsivity and detectivity, the performance of these
detectors is intermediate between cooled photon detectors and uncooled thermal
detectors. The paper contains a brief characteristic of uncooled photodetectors
presently manufactured at VIGO.
Since 1980, when the uncooled quantum 10.6 Ám radiation detectors started to be commercially available, tremendous progress has been achieved. Their performance have been highly improved by the use of newly designed Hg-Cd-Zn-Te semiconductor graded gap structures of optimized composition and doping profiles, the use of the optical immersion and optical resonant cavity principles.
Today a whole family of uncooled, high speed, 10.6 Ám radiation photon detectors is produced. These detectors are available as photo-conductive (PC) and photoelectromagnetic (PEM) mode devices, which differ in detectivity, responsivity, response time, size of the active area, type and size of the housing.
The uncooled 10.6 Ám radiation quantum detectors are characterized by a very high speed, while their responsivity and detectivity approaches or even surpasses that for commercially available, slow thermal detectors. The comparison of the frequency characteristics of currently available 10.6 Ám thermal radiation detectors, liquid nitrogen cooled photoconductors and VIGO's uncooled photodetectors is shown in fig. 1. Let's discuss properties of the last detectors more detail.
Photoconductive (PC) mode detectors require relatively strong electrical biasing constant-current. The highest performance is being achieved with bias power density exceeding 0.1 watt per 1 mm2 area of active element. The voltage responsivity of Vigo's model R005, 1x1 mm2 area photoconductors for 10.6 Ám radiation is over 120 mV/W. The voltage responsivity-width product is essentially constant, so the voltage responsivity of various area devices can be readily estimated as inversely proportional to the width. Detectivity is limited by Johnson noise at inter-mediate and high frequencies. The low frequency (1/f) noise typically decays above 10 kHz. Currently obtained 10.6 Ám detectivity at room temperature is over 107 cmHz1/2/W at frequencies 0.1-300 MHz. Please refer to fig. 2.
The photoconductors exhibit a residual thermal response at very low frequencies (<100 Hz), which may be undesired for some applications.
Recently we have developed a unique monolithic optically immersed photoconductors with highly improved performance. They achieve detectivities of 2x108 cmHz1/2/W, the per-formance typical for slow thermal detectors. At the same time bias power requirements are reduced by a large factor of 7 and 50 for hemispherical and hyperhemispherical immersion, respectively in comparison with conventional non-immersed devices. See fig. 1 and data sheets of the PCI-L model.
Photoelectromagnetic (PEM) detectors do not require electric biasing. Rather, they utilize a magnetic field from an integral permanent magnet to separate electrons and holes created by incident photons absorbed in the semi-conductor. The PEM mode device is a photovoltaic device like a p-n junction photodiode. An increase of PEM detector's parameters was possible by application of modern rare earth permanent magnets and cobalt steel pole pieces together with improved active element processing. The voltage responsivity of Vigo's model PEM-L, 1x1mm photoelectromagnetic detector is over 50 mV/W and detectivity is over 5x106 cmHz1/2/W. Similarly to photoconductors, the responsivity-width products remains approximately constant. The low frequency 1/f noise does not appear, so detectivity is limited by Johnson noise and remains flat over wide frequency band, from DC to very high frequencies.
Recently. the performance of PEM detectors have been improved further by the use of optical immersion. The voltage responsivities of optically immersed Series PEM-I detectors are over 0.3 V/W for 1x1mm2 optical area, with detectivities exceeding 3x107cmHz1/2/W.
NEWEST: PHOTODIFFUSION EFFECT DETECTORS
Photodiffusion effect detectors are new generation of ambient temperature photovoltaic detectors, based on the diffusion photovoltage in the semiconductors. They combine advantages of photoconductors and photoelectromagnetic detectors. Requiring no electric or magnetic bias they exhibit both high performance and high speed. With no flicker noise, they can be simultaneously used for detection of CW and low frequency modulated radiation. For the best performance they can be used with optical immersion. The development of these devices is ongoing and they will be available in the foreseeable future.
The primary reason that uncooled 10.6 Ám CO2 laser photodetectors are of interest at all is their ultra-fast response time. For PC devices, time constant of 1 nanosecond is achieved. The RC time constant, which limits the achievable speed of photodiodes and pyroelectric detectors, is unimportant here due to the low (pF) capacitance of the thin semi-conductor layer device design. Rather, carrier life time and lead inductance seem most important.
In PEM and photodiffusion mode devices, response times can be even shorter than the carrier life time and in fact is set by the carrier diffusion time through the thin HgCdTe layer. The typical response time of Vigo's PEM-L and PEMI-L model response time is below 0.5 nanoseconds and can be further shortened down to about 0.1 nsec, with some expense of performance, however.
Fig. 3. Photoconductive a) and photovoltaic b) detector operation circuits.
The photoconductor operating circuit shown in fig. 3a) consists of the photoconductor in series with load resistor R and dc. bias voltage V, with the output being monitored across the detector. The capacitor C is typically used to eliminate the constant voltage offset from the detector output which is present due to the dc. bias. The value of the capacitor C determines also the low frequency cut-on of the circuit.
PEM and photodiffusion mode detectors are photovoltaic devices and they require no external bias supply. They can be DC coupled to preamplifiers (fig. 3b). Very low noise and wideband preamplifiers are required to achieve in practice the potential performance of both types of detectors.
DAMAGE THRESHOLD AND LINEARITY RANGE
Average power levels of 200 W/cm2 on the detector surface of either PC or PEM devices represent the damage threshold limit due to heating of active elements. For pulsed radiation the damage threshold increases to about 1 MW/cm2 for sub microsecond pulses.
However, above power levels densities of about 100 W/mm2 non-linearity in output can be observed due to increased carrier concentration, resulting in lower responsivity and detectivity. The voltage responsivity may decrease as a result of detector heating or due to increased carrier concentration by optical generation. The first limitation is important for CW and chopped radiation, while the second for short pulses with low repetition rates. Arbitrary selection of 20% maximum deviation from linearity as a threshold results in requirement that the maximum average power density is limited to about 1 W, resulting in output signal for all types of the room temperature photodetectors of a few tens of mV per 1 mm detector length. The maximal output voltage increases to about 1 V per 1 mm length for short, low repetition rate pulse illumination.
The optically immersed detectors due to concentration of radiation on active elements are more vulnerable to laser radiation power. The damage and linearity threshold are reduced by a factor of about 7 and 50 for hemi- and hyper-hemispherically immersed detectors, respectively with corresponding decrease of linearity range. This poses no practical problems since the immersed detectors are used for detection of weak radiation.
Uncooled HgCdTe CO2 laser radiation detectors are especially interesting as heterodyne detectors. Their lower detectivity compared to LN2 cooled devices can be compensated for by the higher power of the local oscillator which can be applied to the uncooled HgCdTe detector. The Vigo's model PCI-L optically immersed photodetectors with detectivity exceeding 2x108 cmHz1/2/W are ideally suited for heterodyne detection. With these types of detectors it should be possible to obtain NEPH as low as 10-19 W/Hz.
The uncooled long wavelength semiconductor photodetectors due to the very short response time and the perfect impedance match to fast electronics are ideal for the detection of the pulsed and high frequency (up to over 1 GHz) modulated CO2 laser radiation. They seem to be especially well suited for the heterodyne detection.
The devices are cheap, rugged and convenient to use. Their unique features together with a low price, should lead to the suppression of slow, at comparable sensitivities and by far less sensitive at high frequencies, thermal detectors.
The practical applications of uncooled 10.6 mm radiation detectors include
detection and monitoring of CO2 laser radiation, laser metrology,
processes control in industrial laser technology, medical laser applications,
scientific research (for example laser radiation interaction with condensed
matter and high temperature plasma), lidars, range-finders and laser
communications, laser threat warning and other military systems. The use of
optical immersion extends application of uncooled detectors to areas where
higher performance is essential.
For more technical details please see: