The EagleEye Optical Spectrum Analyzer (OSA) is a compact unit for measuring the spectral characteristics of lasers. The EagleEye consists of a stable INVAR based confocal high-finesse cavity. One of the cavity mirrors is attached to a piezoelectric actor, allowing a slight variation of the mirror spacing. The variation changes the resonance frequency of the optical cavity and light is transmitted when the incoming laser beam frequency coincides with a cavity resonance.

The EagleEye, like any other conventional OSA device, can analyse a laser spectrum by scanning the cavity. In this mode a triangular shaped voltage waveform is applied to the piezoelectric actor. A record of the transmission will show the frequency structure of the laser beam. This mode is very useful to determine the purity of the single mode operation and to detect e. g. higher transversal modes of the incoming laser beam. In case of a single longitudinal mode laser this will be a chain of resonance peaks separated by a free spectral range of the cavity. The linewidth of the peaks is either determined by the finesse of the EagleEye or by the linewidth of the laser whichever of the two is greater. The finesse of a confocal cavity is in practical application limited to values of 400 .. 500 due to limits set by input beam diameter and surface qualities. Hence, in scan mode the linewidth resolution of the cavity is limited to linewidths in the order of some MHz.

The EagleEye can surpass this limitation by using it’s unique lock, loose, look procedure. To start a meausrement cycle, the EagleEye controller locks the cavity to the laser line. This is done by actively controlling the cavity length so that the laser frequency coincides with a transmission peak’s half maximum position. A change of the laser frequency has the effect of moving up or down on the transmission curve, so any change of the laser frequency will result in a change of the transmitted intensity. For a laser linewidths measurement, the control loop is switched off. Now the cavity spacing is kept constant. Immediately after loosening the lock, the intensity variations are recorded for 100 Milliseconds (look). Any change of transmitted intensity is attributed to a change of laser frequency. Using this approach, a much higher resolution for laser linewidth is possible. The EagleEye can measure linewidths of about 20 kHz.

Components

• INVAR steel cavity with focusing lens
• One cavity optic set, standard wavelength range is 500 .. 1100 nm
• Detector head
• Connection cable
• Controller rack
• Cardanic mount for cavity
• Software package

Specifications

Cavity

• 1.5 GHz free spectral range
• Finesse > 300
• INVAR construction
• Interchangeable optic sets
• Broadband coatings
• Integrated mount for neutral density filters

Detector

• Spectral range 300 nm .. 1100 nm
• Bandwidth 300 kHz
• Variable gain pre-amplifier for photodiode
• Single connector to control rack

Controller

• DSP controller board
• Full speed USB 2.0 port
• Sampling rate of 150 kHz
• Recording time 100 msec
• Optional external control of piezo voltage
• 110 .. 230 line voltage

Software

• LabVIEW™ based control software
• Libraries for LabVIEW support available
• Free update policy
• Windows 2000™, Windows XP™, Windows Vista™ (32 bit) support

Highlights

• Highly stable INVAR cavity
• Interchangeable optics sets
• Broadband optics
• Fully computer controlled system
• Easy to operate, fast alignment
• Display of Fourier-Transform
• Mo extra oscilloscope required
• Immediate display of results

Screenshots

Screenshot of EagleEye in scan mode. The laser source was a narrow linewidth Titanium:Sapphire laser (Matisse TX). The resolution is limited by the finesse of the confocal cavity. In this case a resolution of about 10 MHz is achieved.

Screenshot of an EagleEye in high-resolution measurement mode. The upper trace shows the raw data; the fluctuations of the intensity transmitted by the confocal cavity. The lower trace depicts the Fourier-transform analysis of the above data. The peak at 1000 Hz can be attributed to the piezo controlled etalon of the laser system. The EagleEye software automatically evaluates a linewidth (27 kHz for this measurement).

Screenshot of EagleEye in series mode. When this mode is active the EagleEye performs about 3 measurements of the linewidth per second and displays the result in a scatter graph.