Introduction to GHz Detection Systems

GHz Detection Systems employ a digital storage scope running in equivalent-time mode. The oscilloscope uses a number of trigger events to acquire the required number of samples at different delays corresponding to the trigger. One measurement is made after each trigger. The input signal needs to be repetitive to produce the multiple measurements required for equivalent-time sampling.

GHz Detection Systems employ sequential equivalent-time sampling that involves collection of each of the samples after the onset of the trigger, and commencement of each acquisition at a different time corresponding to the trigger. A measured waveform is reproduced using the measurements recorded in the memory.

Equivalent-time sampling cannot be applied for measurements of single shot signals, due to the requirement for a repetitive signal to realize full bandwidth performance. A reasonably stable waveform can only be achieved when the signal remains unchanged during the signal acquisition. The rapid variation of the signal from shot to shot leads the scope to display the signal with sharp vertical steps. This feature is typically applied to monitor mode-locked YAG lasers as a sign of the instabilities, which may result in uncontrollable Q-switching.

GHz Detection Systems

Oriel Instruments provides a range of detectors covering the visible and NIR spectral ranges. GHz Detection Systems are used for measuring repetitive sub ns pulsed signals, and monitoring the mode-locked laser stability. The response of an optical system may also be measured with these detectors using frequency domain methods. These applications involve the measurement of the variation in the modulation depth and phase of the signal, corresponding to sinusoidally varying stimulus radiation. The following are the key features of GHz Detection Systems:

  • Simultaneous two channel measurement
  • Affordable sampling scope with 2GHz bandwidth
  • Ideally suited to monitor high rep rate ns lasers and mode-locked lasers

Working Principle

A fiber optic transmits the pulsed signal to the ultrafast detector that is attached to the PC board based sampling scope through a BNC connector. Oriel Instruments eliminated cables to avoid pulse broadening caused by the dispersion in cables. The sampling scope is directly mounted in an expansion slot of an IBM PC, allowing transmission of acquired data to the CPU of the user’s PC at a much faster rate than from a standalone sampling scope.

The fiber optic’s input is an SMA or FC connector. Oriel Instruments recommends the use of a fiber coupler for coupling pulsed signal into the optical fiber of the detection system. The fiber is a 50µm core diameter, 1.8m long fiber for all excluding the 2GHz silicon detector, which is provided with a single mode fiber. Four detector options are available from Oriel Instruments for the 76979 and 76980 GHz Acquisition Boards (Table 1).

Table 1. Ultrafast Detector Options

Detector Spectral Range (nm) Sensitivity Bandwidth Rise/Fall Time
Silicon 400 - 1000 0.6 A/W @ 850 nm 350 MHz 1 ns
Silicon 400 - 1000 150 V/W @ 850 nm 2 GHz <200 ps
InGaAs 850 - 1700 0.9 A/W @ 1550 nm >2 GHz <175 ps
IR InGaAs 1200 - 2500 1.3 A/W @ 2400 nm 1 GHz 350 ps

The 76979 and 76980 GHz Acquisition Boards consist of an oscilloscope simulation application that enables controlling the digital scope, just as with a standalone oscilloscope. Oriel Instruments also incorporates a library of LabView™ Virtual Instruments, and 16 and 32 bit DLLs. A 76980 GHz Acquisition Board and 76981 Silicon Detector Package is depicted in Figure 1. The 77799 Fiber Optic Coupler is employed for coupling the light into the fiber.

76980 GHz Acquisition Board and 76981 Silicon Detector Package

Figure 1. 76980 GHz Acquisition Board and 76981 Silicon Detector Package

Specifications

Specifications are listed in Table 2.

Table 2. Specifications

Channels
    76979
    76980
:
:
:

1
2
Rise time (typical) : 175 ps
Bandwidth (0.35/rise time) : 2 GHz
Input ranges : 5 mV/div to 500 mV/div
Input impedance : 50 Ω
Input coupling : DC
Voltage offset : ±2 Volts
Overvoltage without damage : ±5 Volts
Gain accuracy : ±1% at 1 MHz
Bandpass flatness (typical) : ±0.5 dB (10 kHz - 50 MHz, relative to 1 MHz)
RMS noise (typical) : 0.5% FS + 0.7 mV
Vertical resolution
    Equivalent sampling mode
:
:

12 Bits
Buffer size : 1001 samples/channel
Min computer requirements : 80486, 4 MB RAM (8 MB recomended) 5 MB free hard disk space, ISA 16- bit expansion slot
PC bus power requirements : 5 V, 1.5A
+12V, 300 mA
-5V, 200 mA
-12V, 300 mA

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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