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1 Introduction
Making good optical measurements is rarely simple, but it is particularly difficult in very
low light, as anyone knows who’s ever tried it.
Of course, it’s easier if you can get more light somehow, maybe with a brighter laser or
a larger collection aperture. That isn’t always possible, unfortunately. Chemical, biological,
and microelectronic samples are often easily damaged by cranking the illumination power
too high, for instance, and large collecting apertures are bulky and expensive.
Low light naturally requires large feedback resistors in the transimpedance amplifier in
order to reduce thermal (Johnson) noise. Decent-sized photodiodes have a lot of capacitance,
though, which in combination with those large resistors form slow RC time constants that
limit measurement speed, signal-to-noise ratio, or both.
The Hobbs ElectroOptics QL01 is a highly sensitive photoreceiver that achieves shot-noise
limited performance with very dim light, from 10 nanowatts up to a few microwatts, with
a previously unattainable combination of sensitivity and bandwidth. It uses a proprietary
bootstrap architecture to reduce the effect of photodiode capacitance by a factor of over
1000, with sub-nanovolt noise densities. This allows the Model B to offer shot-noise-limited
detection performance at 25 nA out to its full 1 MHz bandwidth, and at 8 nA in 100 kHz.
The Model A trades off a bit of low-light performance for a 7x increase in detector area for
more light collection and ease of alignment.
Low-light applications in solid state physics, spectroscopy, chemistry, biology, and other
fields will see an immediate improvement in their measurements when they switch to the
QL01. For applications with a bit more light available, forthcoming versions C and D will
extend the bandwidth to 3 MHz at 1 MΩ transimpedance. Fiber-coupled versions with
performance from 800-1700 nm are in the works as well.
These instruments are designed to survive the accidents that sometimes happen in a
research lab, with medical-grade power supplies, a thick aluminum box, a stainless steel
mounting thread, and a solid metal BNC output connector securely attached to the box
itself.
The QL01 series are the first of a family of advanced photoreceivers from Hobbs Elec-
troOptics. These are all-new designs based on expertise gained through our own research
and from designing dozens of advanced instruments for our consulting clients. We also do
specials and OEM products, so if you have an unusual requirement, give us a call or send an
email.
Philip C. D. Hobbs Hobbs ElectroOptics/ElectroOptical Innovations, LLC.
Principal 160 North State Road, Suite 203
(914) 236-3005 Briarcliff Manor, NY 10510
