Optical spectrum analyzer (OSA)

An optical spectrum analyzer (OSA) measures how the power of a light source is distributed over wavelength. It passes the light through a narrow optical filter, steps or sweeps the filter across the span of interest and records the power that gets through at each wavelength, so each point of the trace is the power within the filter's width, the resolution bandwidth, usually displayed in dBm. From that trace come most source-level measurements: center wavelength and spectral width, side-mode suppression ratio, the amplified spontaneous emission floor, OSNR, and the gain and noise figure of an optical amplifier.

How a grating analyzer works

Most benchtop analyzers are diffraction-grating monochromators. Light from the input fiber is collimated onto a diffraction grating, which sends each wavelength off at a different angle, and a lens or mirror focuses the dispersed light onto an exit slit in front of a photodetector; rotating the grating sweeps the wavelengths across the slit. The passband is set by the angular dispersion of the grating together with the widths of the image of the input fiber and of the exit slit, and ultimately by the grating's resolving power, $\lambda \mathrm{/}\delta \lambda =mN$ for $N$ illuminated grooves in order $m$. To resolve 0.02 nm at 1550 nm, a resolving power of 77,500, a grating of 900 lines per millimeter used in first order in the Littrow arrangement must be illuminated over 86 mm; double-pass designs send the light across the grating twice, which halves that, and also reduce the stray light that limits how close to a strong line a weak one can be seen. Because the input fiber acts as the entrance slit, the resolution is specified for single-mode fiber input, and a multimode fiber, with its larger core, widens the passband.

Resolution bandwidth

The resolution bandwidth (RBW) is the width of the analyzer's filter, selectable on grating instruments from a few hundredths of a nanometer (5 pm on the finest current instruments) to a few nanometers. In frequency, $\mathrm{\Delta }f=c\mathrm{\Delta }\lambda \mathrm{/}{\lambda }^2$:

Any feature narrower than the RBW is displayed with the shape of the filter. At 0.02 nm, a single-frequency laser with a 1 MHz linewidth appears 2,500 times wider than it is, so an OSA reads the wavelength and power of such a line, and its side modes, but not its width, which is measured by delayed self-heterodyne or heterodyne methods. What a grating analyzer resolves well are features wider than a few RBW: the longitudinal modes of a Fabry-Perot laser, about 1.1 nm apart for a 300 µm chip at 1550 nm; DWDM channels on a 50 GHz grid, 0.40 nm apart; the spectrum of a modulated signal; and broadband sources. Transmitter standards specify spectral width as an rms width for multimode lasers or a −20 dB width for single-mode ones, both measured this way (IEC 61280-1-3).

Reading power and noise

Each point on the trace is the power inside the RBW, and narrow and broadband light scale differently with it. A line narrower than the filter reads its full power whatever the RBW, while broadband light, such as amplifier noise, reads in proportion to the RBW. OSNR is therefore quoted in a reference bandwidth of 0.1 nm (ITU-T G.697): a noise level of −40 dBm read at 0.05 nm RBW is −37.0 dBm in 0.1 nm. The effective noise bandwidth of the real filter differs somewhat from its nominal setting, and analyzers can measure it and use it when they normalize noise readings. The usual OSNR measurement reads the noise between channels and interpolates it under the signal (IEC 61280-2-9), which fails when filters in the link shape the noise or when a signal's spectrum fills its whole channel, as for coherent signals; in-band methods are used then. Amplifier gain and noise figure are measured with an OSA in the same way (IEC 61290-3-1).

Dynamic range

Dynamic range has two meanings: the span between the largest measurable signal and the noise floor, and the close-in dynamic range, how far below a strong line a weak one can still be seen at a given wavelength offset. The second is limited by the skirts of the filter and by stray light scattered inside the monochromator, falls steeply as the offset approaches the RBW, and sets the largest side-mode suppression or OSNR that can be read: a side mode lying below the skirt of the main line is hidden under it. Analyzers specify it at stated offsets and resolution settings, and a measurement near that limit is checked with a narrower RBW, or with a tunable filter that removes the main line before the analyzer. Optical Spectrum Analyzers: Operating Principles and Measurement Configuration gives the settings for each of the standard measurements.

Calibration and accuracy

The wavelength scale is calibrated against absorption lines of known wavelength, either inside the instrument or with an external gas cell, such as the acetylene and hydrogen cyanide references that NIST certifies for 1510 to 1540 nm and 1530 to 1565 nm (Standard Reference Materials 2517a and 2519a); IEC 62129-1 sets out the calibration of OSAs. The power scale is checked against an optical power meter on the same source, since the power integrated over the trace should equal the total power. Grating efficiency depends on polarization, so analyzers depolarize the input or split it into two polarizations, and the residual polarization-dependent response is part of the power uncertainty for polarized sources such as lasers.

Other kinds of analyzer

A scanning Fabry-Perot interferometer resolves the free spectral range divided by the finesse, 7.5 MHz for a 1.5 GHz range and a finesse of 200, but only within that narrow range, so it serves to watch the modes of one laser. Fourier-transform instruments record an interferogram while an interferometer arm moves and compute the spectrum from it; their resolution is set by the maximum path difference, about 12 pm at 1550 nm for 20 cm, and the same principle, with a reference laser, gives the absolute accuracy of a wavemeter. High-resolution analyzers use a swept narrow-linewidth laser, either as the local oscillator of a coherent receiver or as the pump of the narrow gain band of stimulated Brillouin scattering, and resolve features of tens of megahertz or less, fine enough to see the structure of modulated signals that a grating analyzer blurs.

Common questions

What does an optical spectrum analyzer measure?

Optical power against wavelength, with each point of the trace giving the power within the instrument's resolution bandwidth. From the trace come a source's center wavelength and spectral width, its side-mode suppression ratio, the noise floor and OSNR of amplified signals, and the gain of amplifiers.

What is the resolution bandwidth of an OSA?

The width of the instrument's tunable filter, the narrowest wavelength interval it can separate. Benchtop grating analyzers offer settings from a few hundredths of a nanometer to a few nanometers; 0.1 nm corresponds to 12.5 GHz at 1550 nm.

Can an OSA measure laser linewidth?

Not for single-frequency lasers. A grating analyzer's resolution of a few gigahertz is thousands of times wider than the megahertz or kilohertz linewidth of a DFB or external cavity laser, and the line is displayed with the instrument's filter shape. Linewidth is measured by self-heterodyne or heterodyne methods; the OSA measures the side modes and the envelope of multimode lasers.

How is OSNR measured with an OSA?

The signal power is read at the channel peak (or integrated across the channel when it is wider than the resolution bandwidth), the noise level between channels is interpolated to the channel wavelength, and the noise reading is scaled from the resolution bandwidth to the 0.1 nm reference bandwidth. For coherent or tightly filtered signals, whose spectra leave no clean gap between channels, in-band methods are needed instead.

References: D. Derickson (ed.), Fiber Optic Test and Measurement (Prentice Hall, 1998). ITU-T Recommendation G.697, Optical monitoring for dense wavelength division multiplexing systems. IEC 61280-2-9, Optical signal-to-noise ratio measurement for dense wavelength-division multiplexed systems. IEC 61280-1-3, Measurement of central wavelength, spectral width and additional spectral characteristics. IEC 61290-3-1, Optical amplifiers, test methods: noise figure parameters, optical spectrum analyzer method. IEC 62129-1, Calibration of wavelength/optical frequency measurement instruments, Part 1: Optical spectrum analyzers. NIST Standard Reference Materials 2517a (acetylene, 1510 to 1540 nm) and 2519a (hydrogen cyanide, 1530 to 1565 nm), NIST Special Publications 260-133 and 260-137.