Technical Note:
Why is the Noise Spectrum Important?

Understanding the noise spectrum of a laser is important in making a purchase decision, since its noise characteristics will limit the dynamic range of your measurements. Yet it’s often hard to make valid comparisons, because each company specifies the noise differently. That’s why at New Focus™ we specify our swept-wavelength lasers and laser modules in a few different ways so that you can make accurate comparisons with our competitors.

First we specify the amplified spontaneous emission (ASE) at two points away from the carrier by using an OSA at resolution bandwidths of 0.1 nm and 0.2 nm. In addition, we specify the signal-to-ASE ratio, integrated over a wide wavelength range. This is especially important because receivers are insensitive to wavelength and so integrate all the incident power regardless of wavelength.

We measure this integrated dynamic range by observing the spectrum of two cascaded fiber-Bragg gratings with a total rejection ratio of >100 dB and a 0.8-nm window. The fiber-Bragg gratings reject the laser-carrier wavelength while transmitting most of the ASE. The power meter used to measure the ASE has a >90-dB dynamic range. As the laser wavelength is scanned across the fiber-Bragg gratings, the measured rejection ratio is limited only by the noise spectrum of the laser and is a measurement of the ratio of the signal to the total integrated source emission outside the 0.8-nm bandwidth of the filter. This is the achievable wavelength-integrated dynamic range and is a realistic expectation of what you’ll see in your lab.

So when you’re comparing ASE or dynamic range specifications, remember to ask:

1) Was the measurement taken with an OSA? If so, what was the resolution bandwidth? How far away from the carrier? Over what wavelength range?

NOTE: An OSA will slow down your measurement and is not ideal for swept-wavelength measurements. Detection systems should have bandwidths of a few hundred kilohertz.

2) What is the integrated signal-to-noise ratio? How far away was this measurement from the carrier? Over what wavelength range? What method was used?

Optical power transmission through two matched narrow-notch-filter fiber-Bragg-grating reflectors measured with our swept-wavelength tunable laser. The ratio of the signal power to the total integrated ASE-background power outside the 0.8-nm filter width is >70 dB.