Topic report 11/09/01 Optical Spectrum Analyzer (OSA) Speaker: Chieh-Wei Huang Advisor: Sheng-Lung Huang Solid-State Laser Crystal and Device Laboratory

The OSA is used to show power levels as a function of wavelength. The range of the OSA is typically 600 nm to 1700 nm. The OSA has resolution bandwidths typically from 0.08 nm to 10 nm. 2016/6/11 Solid-State Laser Crystal and Device Laboratory2 OSA Intput is “optical signal”

Solid-State Laser Crystal and Device Laboratory Fundamental block figure of OSA 2016/6/11 3 Tunable bandpass filter Position controller input Analog to digital converter Wavelength tuning Horizontal position Vertical position Wavelength (nm) Power (dBm) Photo detector Transimpedance amplifier

Conversion of power (from dBm to mW)(from mW to dBm) x dBm=10*LOG( y mW/1mW)y mW = 10 ^ [ x dBm/10] MilliWatts (y)dBm (x) MilliWatts (y) /6/11 Solid-State Laser Crystal and Device Laboratory4

Tunable band pass filter 2016/6/11 Solid-State Laser Crystal and Device Laboratory5 Transparent prismGrating

Diffraction grating based OSA (single pass) 2016/6/11 Solid-State Laser Crystal and Device Laboratory6 Fiber input Rotating diffraction grating Concave mirror Slit Photodiode Transimpedance amplifier

Photo when initializing 2016/6/11 Solid-State Laser Crystal and Device Laboratory7

Diffraction grating equation 2016/6/11 Solid-State Laser Crystal and Device Laboratory8 θ1θ1 θ2θ2

Rayleigh criterion Two wavelength are just resolved when the maximum of one lies at the first minimum of the other 2016/6/11 Solid-State Laser Crystal and Device Laboratory 9 ResolvedUnresolved Rayleigh criterion Entrance slit, like pinhole, is able to filter out the optical noise we don’t want. Exit slit, can improve the sensing resolution using narrow slit width.

Grating resolving power 2016/6/11 Solid-State Laser Crystal and Device Laboratory10 m: diffraction order N: number of grating blazes : smallest resolvable wavelength difference

Diffraction grating based OSA (double pass) 2016/6/11 Solid-State Laser Crystal and Device Laboratory11 Fiber input First grating Concave mirror Aperature Photodiode Transimpedance amplifier Concave mirror Slit Second grating Concave mirror

Compare with single pass and double pass higher dynamic range of double pass 2016/6/11 Solid-State Laser Crystal and Device Laboratory12

Operation step (1) 2016/6/11 Solid-State Laser Crystal and Device Laboratory13

Operation step (2) 2016/6/11 Solid-State Laser Crystal and Device Laboratory14

Operation step (3) 2016/6/11 Solid-State Laser Crystal and Device Laboratory15

Ocean optics HR4000CG 2016/6/11 Solid-State Laser Crystal and Device Laboratory16

Little question in the experiment 2016/6/11 Solid-State Laser Crystal and Device Laboratory17 But if the OSA is conceptually much like a conventional spectrum analyzer, why is it not used to see modulation? It could be, depending on the relative bandwidths involved. The OSA has resolution bandwidths from 0.08 nm to 10 nm. A common wavelength for optical work is 1300 nm, which is about 230 THz. At this wavelength, a resolution of 0.1 nm is about 18 GHz. So, modulation can be easily seen if its bandwidth is considerably wider than 18 GHz. Optical modulators that have a bandwidth this wide are not yet commonly found. Most RF modulation of a laser is only a few GHz in bandwidth, so all the modulation information falls within the resolution of the OSA. It is like trying to look at 1 kHz sidebands in a 1 MHz resolution bandwidth. Therefore, the OSA does not replace the spectrum analyzer, you need both.

Conversion between nm and Hz 2016/6/11 Solid-State Laser Crystal and Device Laboratory18

Second-order wrong signal 2016/6/11 Solid-State Laser Crystal and Device Laboratory19 Properly setting the slit width (resolution bandwidth) and sensitivity

 Thanks for your attention! 2016/6/11 Solid-State Laser Crystal and Device Laboratory20

Resolvance of grating 2016/6/11 Solid-State Laser Crystal and Device Laboratory21