National Physical Laboratory

Low Drift Etalons

The NPL low drift Fabry-Perot etalon has been designed to provide the best possible frequency stability. Developed as a spin-off from the ion trapping and laser cooling research at NPL, the etalon offers both tuning flexibility and low drift.

NPL low drift etalon

NPL low drift etalon
(showing outer casing and
inner vacuum chamber)


  • Spectral analysis
  • Laser stabilisation
  • Tunable filter
  • Reference marker

Technical Specifications

  • Wavelength range: 630 nm to 1000 nm (other wavelengths available on request)
  • Low frequency drift: << 500 kHz / hour
  • Fast settling time: < 2% overshoot
  • Large scan range: > 10 GHz @ 633 nm
  • Compact design: free spectral range ~ 1500 MHz
  • High resolution: finesse > 100 (higher and lower finesses available on request)
  • System evacuated to eliminate pressure effects: residual pressure < 10-6 mbar
  • System temperature controlled to minimise thermal effects: temperature controlled within 5 mK
  • Low hysteresis

Frequency Drift and Overshoot Characteristics

Attention to detail has enabled the NPL etalon to out-perform conventional reference etalons. The evacuated and carefully temperature controlled etalon forms a reliable frequency reference, improving the performance of any single frequency laser source. Figure 2 shows the frequency drift of a number of sample etalons recorded over a one-hour period. The data was recorded by locking a single frequency laser to the transmission of each of the etalons and monitoring the resulting beat frequency between the test laser and an iodine stabilised HeNe reference. The long-term stability of the NPL etalon is clearly observed.

etalon drift

Drift of various reference etalons with time

The most common source of frequency instability in a tunable etalon comes after a change in the etalon’s length. Following a frequency step, all etalons carry on moving in the same direction after the step. This "overshoot" can last from minutes to hours, seriously degrading the frequency stability of the device. The NPL low drift etalon has extremely low overshoot and settles in a fraction of the time of conventional etalons. Figure 3 shows this behaviour. The use of a longer etalon with accordingly lower free spectral range and sensitivity is sometimes used to reduce the effects of overshoot. The NPL low drift cavity maintains an extremely low degree of overshoot whilst maintaining high sensitivity and scan range.

Etalon overshoot

Monitoring the overshoot of various etalons after a 900 MHz frequency step


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Last Updated: 23 Dec 2014
Created: 12 Aug 2007


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