Configuration Information

Before deciding which spectrometer and configuration you need, please take a few moments to review the following sections which explain some of the important factors that should be considered before making your decision.

If you require any assistance in choosing a specification, please contact us.

Optical Bench Design

The heart of the AvaSpec fibre optic spectrometer is an optical bench with 45, 50 or 75mm focal length, developed in a symmetrical Czerny-Turner design.

Light enters the optical bench through a standard SMA905 connector and is collimated by a spherical mirror. A plane grating diffracts the collimated light, a second spherical mirror focuses the resulting diffracted light. An image of the spectrum is projected onto a 1-dimensional linear detector array.

The optical bench has a number of components installed inside, allowing a wide variety of different configurations depending on the intended application. The choice of these components such as the diffraction grating, entrance slit, order sorting filter, and detector coating have a strong influence on system specifications. Sensitivity, resolution, bandwidth and stray light are explained further in the following paragraphs.

Configuring a Spectrometer for your Application

In the modular AvaSpec design, there are a number of choices to be made on the several optical components and options depending on the application you want to use the spectrometer for.

This section should give you some guidance on how to choose the right grating, slit, detector and other options that will be installed in your AvaSpec spectrometer.

• Wavelength Range

In determining the optimal configuration of a spectrometer system, the wavelength range is first important parameter that defines the grating choice. If you are looking for a wide wavelength range, we recommend to take an A type (300 lines/mm) or B type (600 lines/mm) grating (Grating selection tables in the spectrometer products section). The other important component is the detector choice. There are 9 different detector types each with different sensitivity curves. For UV applications, the new 2048x14 pixel back-thinned CCD detector, the 256/1024 pixel CMOS detectors or DUV enhanced 2048 or 3648 pixel CCD detectors may be the berst choice. For the NIR range, 3 different InGaAs detectors are available.

If you want to combine a wide range with a high resolution, a multiple channel spectrometer  may be the best choice.

• Optical Resolution

If you desire a high optical resolution, we recommend to pick a grating that has 1200 or more lines/mm (C,D,E or F types) in combination with a small slit and a detector with 2048 or 3648 pixels. For example, 10µm slit for the best resolution on the AvaSpec-2048 (see Resolution tables in the spectrometer product sections)

• Sensitivity

When talking about sensitivity, it is very important to distinguish between photometric sensitivity (how much light do I need for a detectable signal?) and chemometric sensitivity (what absorbance difference level can still be detected?)

• Photometric Sensitivity

In order to achieve the most sensitive spectrometer in for example Fluorescence or Raman applications, we recommend the 2048 pixel CCD detector as used in the AvaSpec-2048. In addition, we recommend the use of a DCL-UV/VIS detector collection lens, a relatively large slit (100µm or higher) or no slit and an A type grating. For an A-type grating (300 lines/mm) the light dispersion is minimal, so it has the greatest sensitivity of all grating types. As an option, Thermo-electric cooling of the CCD detector (see product section AvaSpec-2048-TEC) may be chosen to minimise noise and increase the dynamic range at long integration times (60 seconds).

For optimal UV sensitivity, we recommend the back-thinned UV sensitive CCD detector, as implemented in the AvaSpec-2048x14.

For the different detector types, the photometric sensitivity is given in the following table, the spectral sensitivity for each detector can be seen in the curves at the top of this paragraph.

• Chemometric Sensitivity

To detect 2 absorbance values close to each other with maximum sensitivity, you need a high Signal to Noise (S/N) performance. The detector with best S/N performance is the 2048x14 pixel back-thinned CCD detector, next to the 256/1024 CMOS detector in the AvaSpec-256/1024. The S/N performance can also be enhanced by averaging over multiple spectra.

• Timing and Speed

The data capture process is inherently fast with detector arrays and no moving parts. However, there is an optimal detector for each application.

For fast response applications, we recommend to use AvaSpec-USB2 platform spectrometers. When data-transfer time is critical, we recommend to select a small amount of pixels to be transferred with the UBS2 interface. Data transfer time can be enhanced by selecting the pixel range of interest to be transmitted to the PC, in general the AvaSpec-128 may be considered as the fastest spectrometer with more than 8000 scans per second.

The above parameters are the most important in choosing the right spectrometer configuration, please contact us to optimise and fine tune the system to your needs. In the following table, you will see the most common applications and configurations. For more detailed explanation and configurations, please view the applications section of this website.

Quick Reference Guide for Spectrometer Configuration

Application	AvaSpec type	Grating	WL Range (nm)	Coating	Slit	FWHM Res. (nm)	DCL	OSF	OSC
Biomedical	2048	NB	500-1000	-	50	1.2	-	475	-
Chemometry	1024	UA	200-1100	-	50	2.0	-	-	OSC-UA
Colour	128	VA	360-780	-	100	6.4	X/-	-	-
	256	VA	360-780	-	50	3.2	-	-	-
	2048	BB	360-780	-	200	4.1	X/-	-	-
Fluorescence	2048	VA	350-1100		200	8.0	X	-	OSC
Fruit-Sugar	128	IA	800-1100	-	50	5.4	X	600	-
Gemmology	2048	VA	350-1100	-	25	1.4	X	-	OSC
High Resolution	2048	VD	600-700	-	10	0.07	-	550	-
High Resolution	3648	VD	600-700	-	10	0.05	-	550	-
High UV Sensitivity	2048x14	UC	200-450	-	200	2.0	-	-	-
Irradiance	2048	UA	200-1100	DUV	50	2.8	X/-	-	OSC-UA
Laser Diode	2048	NC	700-800	-	10	0.1	-	600	-
LED	2048	VA	350-1100	-	25	1.4	X/-	-	OSC
LIBS	2048FT	UE	200-300	DUV	10	0.09	-	-	-
	2048USB2	UE	200-300	DUV	10	0.09	-	-	-
Raman	2048TEC	NC	780-930	-	25	0.2	X	600	-
Thin Film	2048	UA	200-1100	DUV	-	4.1	X	-	OSC-UA
UV/VIS/NIR	2048	UA	200-1100	DUV	25	1.4	X/-	-	OSC-UA
	2048x14	UA	200-1100	-	25	1.4	-	-	-
NIR	NIR256-1.7	NIRA	900-1750	-	50	5.0	-	1000	-
	NIR256-2.2	NIRZ	1200-2200	-	50	10.0	-	1000	-
	NIR256-2.5	NIRY	1000-2500	-	50	15.0	-	1000	-

Choosing the right Grating

 

 

 

 

 

 

 

 

 

A diffraction grating is an optical element that separates incident polychromatic radiation into its constituent wavelengths.

A grating consists of a series of equally spaced parallel grooves formed in a reflective coating deposited on a suitable substrate. The way in which the grooves are formed separates gratings in two types, holographic and ruled.

The ruled gratings are physically formed into a reflective surface with a diamond on a ruling machine. Gratings produced from laser constructed interference patterns and a photolithographic process are known as holographic gratings.

The fibre optic spectrometer comes with a permanently installed grating that must be specified by the user. In addition, the user needs to indicate what wavelength range needs to reach the detector. Sometimes the specified usable range of a grating is larger than the range that can be projected on the detector.

In order to cover a broader range, a dual or triple beam spectrometer can be chosen. Then master and slave(s) can have different gratings. Similarly, a higher resolution over a wide range can be achieved by using a dual or triple spectrometer.  

For each spectrometer type, a grating selection table is shown in the spectrometer platform section.

The table above illustrates how to read the grating selection table. The spectral range to select depends on the starting wavelength of the grating and the number of lines/mm - the higher the wavelength, the bigger the dispersion and the smaller the range to select.  

By clicking here, the grating efficiency curves are shown. When looking at the grating efficiency curves, please realise that the total system efficiency will be a combination of fibre transmission, grating and mirror efficiency, detector and coatings sensitivities.  

By clicking here, the grating dispersion curves are shown for the AvaSpec-2048.

Selecting Optimal Optical Resolution

 

 

 

 

 

 

 

 

The optical resolution is defined as the minimum difference in wavelength that can be separated by the spectrometer.

For separation of two spectral lines it is necessary to image them at least 2 array-pixels apart.

Because the grating determines how far different wavelengths are separated (dispersed) at the detector array, it is an important variable for the resolution. The other important parameter is the width of the light beam entering the spectrometer. This is basically the installed fixed entrance slit in the spectrometer, or the fibre core when no slit is installed. Slits can be installed with following dimensions: 10, 25 or 50 x 1000µm high or 100, 200 or 500 x 2000µm high.

Its image on the detector array for a given wavelength will cover a number of pixels. For two spectral lines to be separated, it is now necessary that they be dispersed over at least this image size plus one pixel.

When large core fibers are used the resolution can be improved by using a slit of smaller size than the fibre core. This effectively reduces the width of the entering light beam.

The influence of the chosen grating and the effective width of the light beam (fiber core or entrance slit) are shown in the tables in each spectrometer product information.

In the following table, typical resolution can be found for the AvaSpec-2048. Please note that for the higher lines/mm gratings, the pixel dispersion varies along the wavelength range and gets better towards the longer wavelengths (click here to see the illustration).

The resolution in this table is defined as F(ull) W(idth) H(alf) M(aximum), which is defined as the width in nm of the peak at 50% of the maximum intensity (see figure above).

Graphs with information about the pixel dispersion can be found by clicking here so you can optimally determine the right grating and resolution for your specific application.

In combination with a DCL (detector collection lens) or thick fibres, the actual FWHM value can be 10-20% higher than the value in the table. For best resolution small fibres and no DCL is recommended.

Resolution table (FWHM in nm) for the AvaSpec-2048

	Slit Size (µm)
Grating (l/mm)	10	25	50	100	200	500
300l/mm	0.8	1.4	2.4	4.3	8.0	20.0
600l/mm	0.4	0.7	1.2	2.1	4.1	10.0
1200l/mm	0.1-0.2*	0.2-0.3*	0.4-0.6*	0.7-1.0*	1.4-2.0*	3.3-4.8*
1800l/mm	0.07-0.12*	0.12-0.21*	0.2-0.36*	0.4-0.7*	0.7-1.4*	1.7-3.3*
2400l/mm	0.05-0.09*	0.08-0.15*	0.14-0.25*	0.3-0.5*	0.5-0.9*	1.2-2.2*
3600l/mm	0.04-0.06*	0.07-0.10*	0.11-0.16*	0.2-0.3*	0.4-0.6*	0.9-1.4*
	FWHM Resolution (nm)

* = depends on the starting wavelength of the grating; the higher the wavelength, the bigger the dispersion and the better the resolution

Detector Arrays

The AvaSpec spectrometers can be equipped with several types of detector arrays. Currently, we offer silicon based CCD, back-thinned CCD, CMOS and Photo Diode Arrays for the 200-1100nm range. A complete overview is given in the following paragraphs and in the table under the 'Sensitivity heading'. For the NIR range (1000-2500nm), InGaAs arrays are implemented.

CCD Detectors (AvaSpec-2048/3648)
The Charged Coupled Device (CCD) detector stores the charge, dissipated as photons strike the photoactive surface. At the end of a controlled time-interval (integration time), the remaining charge is transferred to a buffer and then this signal is transferred to the AD converter. CCD detectors are naturally integrating and therefore have enormous dynamic range, only limited by the dark (thermal) current and the speed of the AD converter. The 3648 pixel CCD has an integrated electronic shutter function, so an integration time of 10µsec can be achieved.

+ Advantages for the CCD detector are many pixels (2048 or 3648), high sensitivity and high speed.
- Main disadvantage is the lower S/N ratio.

UV Enhancement
For applications below 350nm with the AvaSpec-2048/3648, a special DUV detector coating is required. The uncoated CCD-response below 350nm is extremely poor; the DUV lumogen coating enhances the detector response in the region 150-350nm. The DUV coating has a very fast decay time, typycally in ns range and is therefore useful for fast trigger LIBS applications.

Back-thinned CCD Detectors (AvaSpec-2048x14)
For applications requiring high quantum efficiency in the UV (200-350nm) and NIR (900-1160nm) range combined with good S/N and a wide dynamic range, the new back-thinned CCD detector may be the right choice. The detector is an area detector of 2048x14 pixels, for which the vertical 14 pixels are binned (electronically added together) to have more sensitivity and a better S/N performance.

+ Advantage of the back-thinned CCD detector is the good UV and NIR sensitivity, combined with good S/N and dynamic range
- Disadvantage is the relative high cost

Photo Diode Arrays (AvaSpec-128)
A silicon photodiode array consists of a linear array of multiple photo diode elements, for the AvaSpec-128 this is 128 pixels. Each pixel consists of a P/N junction with a positively doped P region and a negatively doped N region. When light enters the photodiode, electrons will become excited and output an electrical signal. Most photodiode arrays have an integrated signal processing circuit with readout/integration amplifier on the same chip.

+ Advantages for the Photodiode detector are high NIR sensitivity and high speed.
- Disadvantages are limited amount of pixels and no UV response.

CMOS Linear Image Sensors (AvaSpec-256/1024)
These so called CMOS linear image sensors have a lower charge to voltage conversion efficiency than CCD array sensors and are therefore less light sensitive, but have a much better signal to noise ratio.

The CMOS detectors have a higher conversion gain than NMOS detectors and also have a clamp circuit added to the internal readout circuit to suppress noise to a low level.

+ Advantages for the CMOS detectors are good S/N ratio and good UV sensitivity.
- Disadvantages are the low readout speed and relative high cost (1024 pixels).

InGaAs Linear Image Sensors (AvaSpec-NIR256)

The InGaAs linear image sensors deliver high sensitivity in the NIR wavelength range. The detector consists of a charge amplifier array with CMOS transistors, a shift register and timing generator. 3 versions of detectors are available:

256 pixel non-cooled InGaAs detector for the 900-1750nm range
256 pixel 2-stage cooled Extended InGaAs detector for the 1000-2200nm range
256 pixel 2-stage cooled Extended InGaAs detector for the 1000-2500nm range

Sensitivity

The sensitivity of a detector pixel at a certain wavelength is defined as the detector electrical output per unit of radiation energy (photons) incident to that pixel. With a given A/D converter this can be expressed as the number of counts per mJ of incident radiation.

The relation between light energy entering the optical bench and the amount hitting a single detector pixel depends on the optical bench configuration. The efficiency curve of the grating used, the size of the input fibre or slit, the mirror performance and the use of a Detector Collection Lens are the main parameters.

With a given set-up, it is possible to make measurements over around 6-7 decades of irradiance levels. Some standard detector specifications can be found in the following table. As an additional option, DCL cylindrical detector collection lens can be mounted directly on the detector array. The quartz lens (DCL-UV for AvaSpec-2048/3648) increases the system sensitivity by a factor of 3-5, depending on the fibre diameter used.

In the following table, the overall sensitivity is also given for the detector types currently used in the UV/VIS AvaSpec spectrometers as output in counts per ms integration time for a 16-bit AD converter. To compare different detector arrays, we have assumed an optical bench with 600lines/mm grating and no DCL. The entrance of the bench is an 8µm core diameter fibre connected to a standard AvaLight-HAL halogen light source. This is equivalent to ca. 1µWatt light energy input.

In the 2nd table below, the specifications for the NIR spectrometers is given.

Detector specifications (based on a 16bit AD convertor)

Detector	TAOS128	HAM256	HAM1024	SONY2048	TOSHIBA3648	HAM2048x14
Type	Photo diode array	CMOS linear array	CMOS linear array	CCD linear array	CCD linear array	Back-thinned CCD array
# Pixels, pitch	128, 63.5µm	256, 25µm	1024, 25µm	2048, 14µm	3648, 8µm	2048x14, 14µm
Pixel width x height	55.5 x 63.5	25 x 500	25 x 500	14 x 56	8 x 200	14 x 14 (total height 196µm)
Pixel well depth (electrons)	25000	4000000	4000000	40000	120000	250000
Sensitivity V/lx.s	100	22	22	240	160	200
Sensitivity Photons/count@600nm	100	440	440	40	60	50
Sensitivity (AvaLight-HAL, 8µm fibre) in counts/µW per ms integration time	4000 (AvaSpec-128)	120 (AvaSpec-256)	120 (AvaSpec-1024)	20000 (AvaSpec-2048)	14000 (AvaSpec-3648)	16000 (AvaSpec-2048x14)
Peak wavelength	750nm	500nm	500nm	500nm	550nm	650nm
Signal/Noise	500:1	2000:1	2000:1	200:1	350:1	500:1
Dark noise (counts RMS)	60	28	60	35	35	50
Dynamic range	1000	2500	2500	2000	2000	1300
PNRU**	±4%	±3%	±3%	±5%	±5%	±3%
Wavelength range (nm)	360-1100	200-1000	200-1000	200*-1100	200*-1100	200-1160
Frequency	2MHz	500kHz	500kHz	2MHz	1MHz	1.5MHz

* = DUV coated

* = Photo response non-uniformity = maximum difference between output of pixels when uniformly illuminated, devided by the average signal

NIR Detector specifications

Detector	NIR256-1.7	NIR256-2.2	NIR256-2.5
Type	Linear InGaAs array	Linear InGaAs array with 2 stage TE cooling	Linear InGaAs array with 2 stage TE cooling
# Pixels, pitch	256, 50µm	256, 50µm	256, 50µm
Pixel width x height	50 x 500	50 x 500	50 x 500
Pixel well depth (electrons)	16000000	1500000	1500000
Sensitivity (AvaLight-HAL, 8µm fibre) in counts/µW per ms integration time	350	250	200
Peak wavelength	1550nm	2000nm	2300nm
Signal/Noise	4000:1	1200:1	1200:1
Dark noise (counts RMS)	12	40	40
Dynamic range	5000	1600	1600
PNRU**	±5%	±5%	±5%
Wavelength range (nm)	900-1750	1000-2200	1000-2500
Frequency	500kHz	500kHz	500kHz

* = Photo response non-uniformity = maximum difference between output of pixels when uniformly illuminated, devided by the average signal

Stray Light and 2nd Order Effects

Stray light is radiation of the wrong wavelength that activates a signal at a detector element. Sources of stray light can be:

• Ambient light
• Scattering light from imperfect optical components or reflections of non-optical components
• Order overlap
   

Encasing the spectrometer in a light tight housing eliminates ambient stray light.             

When working at the detection limit of the spectrometer system, the stray light level from the optical bench, grating and focusing mirrors will determine the ultimate limit of detection. Most gratings used are holographic gratings, known for their low level of stray light. Stray light measurements are being carried out with a laser light, shining into the optical bench and measuring light intensity at pixels far away from the laser projected beam. Other methods use a halogen light source and long pass or band pass filters.

Typical stray light performance is <0.05 % at 600nm, <0.10 % at 435nm and <0.10 % at 250 nm. 

Second order effects, which can play an important role for gratings with low groove frequency and therefore a wide wavelength range, are usually caused by the grating 2nd order diffracted beam. The effects of these higher orders can often be ignored, but sometimes need to be taken care of. The strategy is to limit the light to the region of the spectra, where order overlap is not possible.

Second order effects can be filtered out, using a permanently installed long-pass optical filter in the SMA entrance connector or an order sorting coating on a window in front of  the detector. The order sorting coatings on the window typically have one long pass filter (590nm) or 2 long pass filters (350nm and 590nm), depending on the type and range of the selected grating.

In the following table, a wide range of optical filters for installation in the optical bench can be found. The use of following long-pass filters is recommended for the following gratings: OSF-475 for grating NB and NC, OSF-515/550 for grating NB and OSF-600 for grating IB.

In addition to the order sorting coatings, we implement partial DUV coatings on Sony 2048 and Toshiba 3648 detectors to avoid second order effects from UV response and to enhance sensitivity and decrease noise in the Visible range.

This partial DUV coating is automatically automatically for the following grating types:

• UA for 200-1100nm, DUV400, first 400 pixels coated only
• UB for 200-700nm, DUV800, first 800 pixels coated only

Filters installed in the AvaSpec spectrometer series

OSF-385	Permanently installed 1mm order sorting filter @371nm
OSF-475	Permanently installed 1mm order sorting filter @466nm
OSF-515	Permanently installed 1mm order sorting filter @506nm
OSF-550	Permanently installed 1mm order sorting filter @541nm
OSF-600	Permanently installed 1mm order sorting filter @591nm
OSC	Order sorting coating with 590nm longpass filter for gratings VA, BB (>350nm) and VB
OSC-UA	Order sorting coating with 350 and 590nm longpass filter for grating UA in AvaSpec-1024/2048/3648/2048x14
OSC-UB	Order sorting coating with 350 and 590nm longpass filter for gratings UA or BB (<350nm) in AvaSpec-1024/2048/3648/2048x14

Spectrometer Platforms

The AvaSpec Spectrometer System is available in different platforms, consisting of different electronics, optical benches and detectors. The following table gives an overview of the different platforms, the nomenclature and technical specifications.

The AvaSpec spectrometer platform was designed to enable applications in the various fields. The concept in the R&D phase was to design a platform, based on a powerful microprocessor system, with stand-alone capability, multi-channel simultaneous readout, digital in and outputs as well as USB and RS232 to allow easy interfacing with or without computer environment.

Fast selection guide

Product	Electronics	Optical Bench	Detector	Housing
AvaSpec-128	AS161 with USB	AvaBench-45, all gratings 360-1100nm	TAOS128	STD single channel
AvaSpec-128USB2	AS5216 with USB2	AvaBench-45, all gratings 360-1100nm
AvaSpec-256	AS161 with USB	AvaBench-45, all gratings 200-1100nm	HAM256	STD single channel
AvaSpec-256USB2	AS5216 with USB2	AvaBench-45, all gratings 200-1100nm
AvaSpec-1024	AS161 with USB	AvaBench-75, all gratings 200-1100nm	HAM1024	STD single channel
AvaSpec-1024USB2	AS5216 with USB2	AvaBench-75, all gratings 200-1100nm
AvaSpec-2048	AS161 with USB	AvaBench-75, all gratings 200-1100nm	SONY2048	STD single channel
AvaSpec-2048USB2	AS5216 with USB2	AvaBench-75, all gratings 200-1100nm
AvaSpec-3648USB2	AS5216 with USB2	AvaBench-75, all gratings 200-1100nm	TOSHIBA3648	STD single channel
AvaSpec-2048x14USB2	AS5216 with USB2	AvaBench-75, all gratings 200-1100nm	HAM2048x14	STD single channel
AvaSpec-NIR256-1.7	AS5216 with USB2	AvaBench-50, grating 900-1750nm	HAMNIR256-1.7	STD dual channel
AvaSpec-NIR256-2.2	AS5216 with USB2	AvaBench-50, grating 1000-2200nm	HAMNIR256-2.2	Desktop
AvaSpec-NIR256-2.5	AS5216 with USB2	AvaBench-50, grating 1000-2500nm	HAMNIR256-2.5	Desktop
AvaSpec-xxxx-2 (xxx = 02/256/1024/2048)	AS161 with USB, 2 channels	AvaBench-45/75, all gratings 200-1100nm	TAOS128, HAM256/1024, or SONY2048	STD dual channel
AvaSpec-Multichannel as Desktop or Rackmount	AS161 with USB1 or AS5216 with USB2	AvaBench-45/75, all gratings 200-1100nm	All detectors	Desktop or Rackmount