Motivation:
We can see many papers and journals, which mention that photonic crystal fibers have low environmental fluctuation dependence including temperature. However, no paper has shown experimental results of this low temperature dependence.
Moreover, photonic crystal fiber’s exceptionally small dependence on environmental change (temperature, in this paper) can open new possibility of building very robust temperature insensitive fiber Sagnac loop interferometers.
Experiments:
A Sagnac loop configuration was built using a standard 50/50 fused fiber coupler with the SMF-28 fiber pigtails and a 1-meter long commercial elliptical-core PM-PCF fiber from the Thorlabs (PM-1550-01, loss < 1.5 dB/km) (see Fig.1). The two ends of fiber coupler were fusion spliced to the PM-PCF forming the Sagnac loop. A portion ( L’ = 0.6m) of both PM-PCF and PMF from the two different Sagnac loop was placed in a temperature-controllable furnace (Yamato, DX 300). An optical incoherent broadband CW light source (HP 83437A) was used as the light input. The light is launched into the single mode 50:50 fiber coupler via a commercial single mode fiber (SMF-28) patch cord, then Temperature dependence was measured from 40 °C to 240 °C for both Sagnac loops in steps of 10 °C. The each output of the interferometer was connected to an optical spectrum analyzer (Ando AQ6135A, resolution=0.05 nm). Another Sagnac loop using an elliptically stress-induced PMF from 3M (FS-PM-7811) with the same length was built for comparison.
 
Figure 1. Schematic of experimental setup and the structure of photonic crystal fiber.
Results
Fig. 2 shows filtering spectra and its shift due to temperature change for the PMF based Sagnac loop.
Figure 2. Peak shifts of PMF-Sagnac. (Dotted line is for the eye.)
Temperature increment in steps of 2 ° C was also measured for the PM version due to the large temperature dependence of the PM version. (see Fig. 3)
Fig. 3 shows filtering spectra and its shift due to temperature change for the PM-PCF based Sagnac loop.
Fig. 3. Peak shifts of PM-PCF-Sagnac. (Dotted line is for the eye.)
Fig.4 shows experimental and theoretical results of the peak shift for both PM-PCF and PMF based Sagnac filters.
Fig. 4. Temperature dependent peak shift for PMF and PM-PCF expressed in units of Δλ .
PM-PCF Sagnac shows very little temperature dependence compared to PMF. The calculated birefringence of the PM-PCF measured exhibited 35 times less temperature dependence than that of the standard PM fiber.
In PM-PCF, the core and the cladding of fiber are made of the same pure-silica, thus when temperature is applied, it does not exhibit differential thermal expansion of the core and the cladding, which result in the internal birefringence. Furthermore, pure silica has a very low thermal expansion coefficient and the change in the birefringence due to the thermal expansion of photonic crystal structure is insignificant.
Reference
D. Kim and J. U. Kang, "Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with reduced temperature sensitivity," Opt. Express 12, 4490-4495 (2004),
http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-19-4490 |
