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The lead sulfide detector has been chosen for the NIR instrument because it provides good detectivity (defined in section 4.5.1) for wavelength ranges from 1000 to 2500 nm, and particularly so in 1900 to 2500 nm. Therefore it is ideal for the required bioprocess application. The detector used is a single element and has an active area of 3x3 mm. In order to increase the detectivity, a two stage thermal electric (TE) cooler is used. The detector was supplied together with temperature controller, bias voltage power supply and amplifier circuits that are integrated to a single unit (IRI 2700 PbS, TC-328 controller, Graseby Infrared, Orlando, USA). The detector is required to operate with chopped optical signal, and incorporates AC coupled electronic amplification circuits. This was because the 1/f noise caused by the bias current makes them unsuitable for DC operation and the AC coupled electronics suppresses this noise. The following figure illustrates the detection system implemented.

N ear Infrared S pectroscopy Technique f o r Bioprocess Monitoring and Control Chapter 4 C h o p p er Light so u rc e O ptics S ignal Integrator NIR sp ectru m P b S d etec to r S am p le

S p ectro g rap h _{Pre-am plifier} & amplifier Bias voltage

Intel

Temp, control S ta g g e r

Fig 4.7 Schematic diagram of the complete PbS detection system. The mirrors used in the spectrophotometer have been simplified and illustrated by a simple optics block. The detector operates with chopped light source and this is introduced before the sample cell. After the interaction and light through the spectrograph, the NIR spectrum is collected by the PbS detector. The PbS is operated with a bias voltage and under temperature controll, the detected NIR energy is transformed to electrical signal. The electrical signal is then integrated by a PC and the NIR spectrum is stored.

The constant optical energy is chopped by a mechanical rotating chopper wheel as also seen in the design layout earlier (figure 4.1). This modulates the optical source providing pulsing energy for detection. The lead sulfide detector generally has lower noise with higher modulation frequency, however, the detection response reduces as

modulation frequency increases. The modulation frequency has been optimised

between detector noise, signal detectivity and avoid problems from the natural modulation frequencies o f noise sources (50 Hz mains power source and its harmonics). A chopped or modulation frequency o f 375 Hz has been chosen.

The detector is biased by ±60 volts DC and a two stage thermal electric (TE) cooler

is build into the detector package. During operation it is set to operating at a

temperature o f 263°K (-10°C ). The heat generated from the cooler is dissipated by a heat sink which is m^:'unted on the back o f the detector package. The advantage o f em ploying a TE cooler is discussed in section 4.5.3.

The output from the PbS detector is AC coupled electrical signal that has amplitude

N ear In frared S pectroscopy Technique f o r B ioprocess M onitoring a n d C ontrol C hapter 4

amplified in the supplied detector electronics. This has a low output impedance with maximum ±10 volts output. The output from this complete detection system is then feed into a data acquisition board (AT-MIO-16XE-50, National Instrument, Berkshire, UK) and interfaced to a IBM compatible 80486 personal computer. The PC will act as a signal integrator that converts optical pulse energy signal to a numerical units before further data analysis.

In the following sections, the basic theory of detection system and the practical aspects of the selected detector system will be described. Firstly, a brief review of figures of merit of a detector are given, then the characteristics of the lead sulfide detector which led to its use are highlighted. Followed are the temperature characteristics of the detector and compensation techniques. Then there is a review of detector noise and the specific detection system adapted in order to minimise noise detection.

4.5.1 PbS detectors and figures of merit

Detectors are described by certain figures of merit. The figures of merit are usually functions of wavelength and temperature and may be affected by detector size, modulating frequency, bias voltage and the gain of any internal amplifier. Three commonly used figures of merit are given below.

Noise equivalent power CNEP)

NEP is the radiant flux in watts necessary to given an output signal equal to the r.m.s. noise output from the detector. The flux may be either continuous or sinusoidally modulated.

The response is assumed to be linear down to the noise level. NEP values should be stated at a specified wavelength, modulation frequency, detector area, temperature and detector bandwidth. Detector bandwidth is usually chosen as 1 Hz and NEP is frequently quoted in watts NEP is the most commonly used version of Noise

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Detectivity (D)

Detectivity, D is the reciprocal of NEP. This gives a figure o f merit which is larger for more sensitive detectors.

Normalised Detectivity (D*)

For most detector DAd^^^ is constant, i.e. the detectivity varies inversely with the square root of the area of the detector. This is because the electrical noise power is usually proportional to the detector area Ad, and current or voltage, which provide a measure of that nose, are proportional to the square root of power. Similarly, because most detector noise is white noise, and the white noise power is proportional to Af (where Af is the electrical bandwidth), the noise signal is proportional to Af^^^. D* is defined to allow comparison of different types of detectors independent of the detector area and bandwidth.

D* = D (Ad‘'^

= D (A c A f)“ ' [4.9]

= (An Af)'” [4.10]

NEP

The units of D* are cm Since D*, like NEP, is a function of wavelength and modulation frequency, it is common to see the normalised spectral responsivity D*(A), or D*(A,f,Af). Figure 4.8 illustrates the D* of a number of detectors.

In comparison with other detectors (see figure 4.8), lead sulfide PbS is ideal for this application of NIR spectroscopy because its detectivity response covers the wavelengths of interest and it is for this reason PbS detector is used in this spectrophotometer. The PbS normalised detectivity (D*) is function of modulation frequency and operating temperature. In the following sections, the selection of bias voltage, operating temperature and modulation frequency are presented.

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Diode InGaAs \ D iode.

Ideal case for photovoltaic detector limited by 293K background PbS @ 243K CCD ^ PbSe @ 243K HgCdZnTe tT @ 243K Pyroelectric 10“ Thermopile 10* _HgCdZnTe ^ @ 2 9 3 K PDA 10* 6 8 10 12 2 4 W avelen gth /^m

Fig 4.8 Normalised detectivity (D*) of various detectors are presented. This emphases PbS has the appropriate detectivity at the wavelength of interest.

4.5.2 PbS detectors bias supply voltage

PbS detector is fabricated by chemically depositing polycrystalline film on a quartz substrate. Gold electrodes are plated to the edges of the film to provide electrical contact and the whole assembly is sealed inside a package with an appropriate window, quartz or sapphire, as illustrated in figure 4.9.

PbS active area Clad steel wire Gold Ii 1/ electrode _ Quartz or sapphire window Quartz substrate Hermetically sealed

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A lead sulfide detector is a form o f photoconductive detector which absorbs incident photons to produce free charge carriers. These change the electrical conductivity of the detector. A bias voltage is applied (figure 4.10b), causing a current to flow which is proportional to the photon irradiance.

! N IR e n e r g y ^ D e p o s it e d P h o t o c o n d u c t o r o V S u b s t r a t e + v e B ia s 'Load P re - \ A m plifier N IR e n e r g y P b S ^ P h o t o c o n d u c t o r - v e B ia s

Fig 4.10 a) Schematic of a photoconductive detector, b) PbS simple circuit arrangement for conversion of NIR energy to electrical signal before pre-amplifier.

A bias voltage o f ±60 volts and loading resistance o f 1 MC2 (figure 4.10b) have been used. This follow ed the detector manufacture’s data specification for the 3x3 mm active detection area (figure 4.11).

1000

100

0 3

0.0001 0.001 0.1 0.01 1