Product/Application Specific FAQ

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10 Things Spectrometer Manufacturers May Not Tell You...

1. Resolution is not always the key criteria when specifying a spectrometer. Rarely in nature to you find two substances that have peaks which are ‘tightly’ close to each other, and you be the one tasked to resolve them. Think again before you specify that high spectral resolution or the high pixel count detector. 2. Throughput is not only f/#, but also needs to be matched to the spectrometer input. The f/# has no information on source, image or detector. If the f/# of the beam from the fiber is increased to match the acceptance f/# of the spectrograph/monochromator, the beam size on the slit increases and less light enters the spectrograph monochromator. The term optical “etendue” gives truer sense of what is attainable and what is not as is described in the following drawing.

BaySpec_F2_Optics

3. It pays to be cool. Cooling the CCD/InGaAs reduces the dark current dramatically. The dark noise typically drops to half when the temperature of the CCD detector chip drops every 10°C for CCD or 7°-8°C for InGaAs. Advancements in semiconductor technology over the last 30 years has increased the availability of thermal electrical cooler chips, created improvements in hermetical sealing and long life vacuum processes, and allowed for the development and use of thermoelectrically cooled detectors as a replacement for cumbersome Liquid Nitrogen based approaches. 4. All detectors are not the same. As an example, both the Sony ILX511 and Hamamatsu S10420 have 2048 pixels, but difference in noise is 10x, and the detector sensitivities should be matched for the spectral range of interest. 5. Size matters. A larger optical bench offers higher resolution and better stray light reduction due to its longer focal length compared to smaller benches. Then again, the application requirements will ultimiately determine the trade offs on size and performance. 6. Some systems are designed to work only with a fiber optic probe. The coupling loss is typically ~50% from fiber end A to fiber end B, so unless necessary it should be reconsidered in favor of direct sampling when working with weak signals. 7. Sensitivity is the smallest detectable change in a relative signal. More difficult to specify is the lowest detectable amount of light or how many photons are needed for the detector electronics to record a change. Careful consideration should be given to the light intensity of a light source and the coupling efficiency. Furthermore, these parameters are all wavelength-dependent. 8. Most customers do not think they have much choice when selecting wavelength range and chose based on standard ranges offered by most manufacturers. As BaySpec manufacturers the gratings in-house, we have total control over the wavelength range design and can offer a customized range that will optimize the resolution over the range of interest. 9. Visualizing, processing and managing spectroscopy data is an important consideration when considering software package. An open system is necessary for most OEM applications, allowing the user to have control of method development, building libraries while providing a full software development kit with dll for modularization of code, code reuse, efficient memory usage, and reduced disk space. 10. Design of Engineering (DOE) is an important consideration when considering an OEM spectrometer manufacturer. All matters related to the final product use should be “designed in” from optics, mechanics, electronics and software for its intended use. The End of Life (EOL) should also be considered particularly for field-based industrial applications.   BaySpec’s years of experience in optical design and broad technology manufacturing platform allow for optimized solutions for performance, reliability and price. With over 35,000 spectral engines of all types in the field representing billions of device hours. For more information, please contact BaySpec at +1(408)512-5928 or info@bayspec.com.

What wavelength range can BaySpec's products cover?

With its in-house Volume Phase Grating™ (VPG) manufacturing capabilities, BaySpec covers a very wide spectral range in all its devices, from 300 nm to 2800 nm.

How is spectral resolution defined?

There are different definitions regarding to the spectral resolution:

(a). Display Spectral Resolution: decimal points in the displayed wavelength value. We have 3 decimals in nanometer, which is 1 picometer (pm) resolution for displayed wavelength value.

(b). Readout Spectral Resolution: this is the minimum value of the wavelength variation which can be detected and reported by this device. For example, If you input a tunable laser signal to the FBGA and tune the laser wavelength by 1pm step, you will see the FBGA reported wavelength value following the tunable laser with each 1pm step.

(c). Physical Wavelength Resolution: this is the minimum wavelength spacing between two signal peaks which can still be distinguished as two peaks.

Normally, the physical wavelength resolution is not important in FBGA application. The most important thing in FBGA application is the readout spectral resolution. We have 1pm readout spectral resolution in our FBGA products and which is achieved by a special arithmetic in the firmware.

What is the difference between FT Raman and the dispersive Raman that BaySpec manufacturers?

BaySpec instruments are dispersive, using a grating and a detector array, which record the dispersion of wavelengths in one acquisition. FT-Raman instruments use a single-point detector and a moving interferometer to scan the wavelengths, which take much longer time for measurement of a spectral range.  Our dispersive spectrometers have no moving parts and record the entire Raman spectrum simultaneously. In addition, the throughput for the dispersive spectrometer is greater than for FT-Raman. For microscopic applications, dispersive Raman provides a high spatial resolution which is impossible for FT-Raman system, enabling high-resolution Raman chemical imaging from dispersive Raman microscopes. The only advantage an FT-Raman has is that it can typically achieve higher spectral resolution than the dispersive system. However, for Raman spectroscopy, most samples in nature can be resolved with 4-10 wavenumber (cm-1) resolution offered on our dispersive systems.

What are BaySpec's FBG Interrogation Analyzers?

A Fiber Bragg Grating Interrogation Analyzer (FBGA) is a miniature optical spectral analyzer (OSA) designed for real-time detection and analysis of spectral signals reflected from fiber Bragg grating sensors. FBGAs are adapted from ruggedized optical channel performance monitoring (OCPM) devices, and feature very high reliability, compact size, and extensive field-deployment lifetime data. Some version of the BaySpec FBGA also feature an internal reference source (FBGA-IRS).

How can BaySpec's FBGA-IRS measure wavelength with absolute accuracy of ±5pm?

BaySpec’s FBGA-IRS design implements an internal wavelength reference source (IRS) as an absolute wavelength standard. It has a wavelength accuracy of +/-5 pm over -10 to + 70 °C, while maintaining this over the full life of the product. No regular calibration is needed as the IRS is used to ‘calibrate’ the FBGA in real time, using an algorithm to make very accurate measurements in the sub-pixel accuracy level. Customers can have steady wavelength accuracy over any time period, any temperature, and anywhere, owing to the portable form factor and ruggedized design. The absolute accuracy of ± 5pm is comparable to that of a laboratory-grade wavelength meter (typically >US $40k). Thus, BaySpec’s FBGA-IRS thus offers significant cost savings with ease of use. It is designed for use not only as a high accuracy wavelength meter in the laboratory, but many demanding field applications.

How to define optical signal to noise ratio (OSNR) in OCPM?

In OCPM, we define OSNR of a channel as an absolute ratio of the clean optical signal power P(signal) to noise power P(noise), measured in units of dB, i.e.,

OSNR = 10 × log10[P(signal)/P(noise)]

In practice, we approximate P(signal) = P(mixed) – P(noise) since the clean optical signal is not obtainable, where P(mixed) is the total power measured at the corresponding channel wavelength. For detailed OSNR measurement, please refer to OSA documents.

How can BaySpec's OCPMs discern different modulation schemes (10/40/100, etc.)?

This is accomplished by a single module which will work with single or dual modulations. The principle function is based on the first optics principle, as any laser beam would work in such a way that when the modulation speed is higher, the spectrum of that wavelength would be broadened – a phenomena called spectral linewidth broadening. As modulation changes, the device can judge the spectral broadening using that info to derive and identify different signals.

The above principle requires an optical channel performance monitor (OCPM) with the following features:
1) fast response time
2) stable wavelength accuracy
3) ability to measure the whole spectra, as a spectrometer would
4) on-board data processing capacity

What information is needed to build a customer-designed EDFA?

In order for our optical engineering team to build an erbium-doped fiber amplifier (EDFA), we need to know the following requirements:

Single-Channel

  • Operating wavelength range
  • Input power
  • Required gain
  • Noise figure

Multi-Channels

  • Operating wavelength range
  • Total input power
  • Channel numbers
  • Required gain
  • Gain flatness
  • Noise figure
Would it be possible to design an EDFA with a gain that could be flattened at different levels?

Yes, if a dynamic gain equalizer is employed. But an EDFA with a static gain flattening filter can only be flattened at one gain level.