On-Chip Integrated Interrogator Based on Silicon Microring Resonator

Chen Qiu*, Jianyi Yang, and Xiaoqing Jiang Department of Information Science and Electronic Engineering

Zhejiang University Hangzhou, China

* zj u.qiuchen@gmail.com

Abstract-An on-chip interrogator for wavelength-modulated optical sensors is developed based on a thermal tunable silicon microring resonator. The center wavelength of the sensors can be readout from the tuning power corresponding to the peak optical output intensity of the system by scanning the resonant wavelength of the microring filter. Both on-chip (microring) and off-chip (FBG) wavelength-modulated sensor are interrogated by our microring interrogator.

Keywords-Interrogator, Sensor, Microring, Silicon

I. INTRODUCTION

Both on-chip and off-chip wavelength-modulated optical sensors such as fiber Bragg grating and integrated microring sensors are widely used for physical, chemical and bio­chemical sensing applications. These sensors can achieve high sensitivity and detection limits. The interrogation method for the wavelength-modulated sensors is also very important. A compact, low cost and high accuracy interrogator needs to be developed. Here in this paper, we demonstrated an on-chip interrogator for wavelength-modulated optical sensors. The interrogator is developed based on the thermal tunable silicon microring resonator. Both on-chip and off-chip sensors are interrogated by the integrated microring interrogator.

II. THEOR Y AND EXPERIMENTS

The sensing system with the on-chip microring interrogator is illustrated in Fig. l. The system consists of a broadband light source (1520 nm-1570 nm), a wavelength-modulated optical sensor, a signal generator, a computer for controlling and analyzing, a photo-detector and an integrated thermal tunable optical microring add-drop filter. Light intensity from the drop port of the microring filter is detected by the photodetector. The signal generator is used to apply the scanning electrical signal on the thermal electrode for moving the resonant wavelength of the microring filter. Assume the power from the broadband light source has a spectral distribution Pj(A), the reflection or transmission from the sensor is Tsens(A) , and the drop port transmission of the microring filter is TD(A). Like the interrogation principles of the tunable A WG as illustrated in [1] , if the resonant wavelength of the microring filter is tuned by b.A, the output intensity from the drop port of the microring filter can be written as

Broadband Light Source

Photo­detector

Wavelength­modulated Sensor

Computer

Figure 1. Setup for the wavelength interrogation of optical sensors by a tunable silicon microring add-drop filter.

P(�A) = f �(A)' Rsens(A)' TD(� - A)dA (1)

Since the tuned wavelength b.A is related to the tuning power applied on the microring filter and the output power is the maximum when the resonant wavelength coincides with the center wavelength of the sensor, then the center wavelength of the sensor can be easily readout from the applied tuning power corresponding to the maximum or minimum output optical intensity.

Figure 2. Top view of the fabricated thermal tunable microring add-drop filter.

o

E m -5 -c-C (1)-N C

= 0 CIS -iii -10 E tI) o 'E Z tI) C -15 �

I-

1545 1546 1547 Wavelength (nm)

Figure 3. Drop port transmission of the tunable micorirng filter.

The microring interrogator is fabricated by CMOS compatible processes on an 8 inch SOl wafer. A TiN microheater is also fabricated on the resonator for thermal tuning as shown in Fig. 2. The waveguide cross-section is 450 nrnx220 nrn and the radius of the microing is 10 Ilm, which ensures a 9 nrn tuning range. The drop port response of the microring filter is shown in Fig. 3. The resonant wavelengths of the microring filter red-shift with the increasing of the tuning power. The relationship between the resonant wavelength of the filter and the applied thermal power can be expressed as

Ares =1545.26 nm+O.104 nmlmW· Pheat (2)

A FBG temperature sensor and an on-chip microring sensor are interrogated by the tunable microring filter respectively. The interrogation results are shown in Fig. 4 and 5. The Bragg wavelength of the FBG temperature sensor is varied with the temperature_ We interrogated the FBG sensor under different temperature by the integrated microring interrogator as shown in Fig. 4. The on-chip microring sensor is based on an all-pass microring resonator as demonstrated in [2] and the output port of the sensor is connected with the microring interrogator by a bus waveguide on the same chip. Both of these two sensing schemes show good accuracy with comparing with the results measured by the optical spectra analyzer.

1550.6 1,========::::;---1

E E. 1550.4 .s= 0, s:::: (I) 1550.2 � �

1550.0

• Measured • Interrogated

- Linear Fit of Measured.1 ./

- Linear Fit of I nterrOgat�

---� irn ...... .! ........ .:

Heating Power

40 60 80 Temperature (0G)

Figure 4. The interrogated and measured (by optical spectra analyzer) Bragg wavelength of the FBG sensor with the FBG under different temperature.

Inset shows the output intensity with different heating power applied on the microring interrogator.

1545.0 rr=.=M= e=a=s=ur=e =d ====::::;-------,

E 1544.5 .:. � g, 1544.0 ., 0; iU :i: 1543.5

o

• Interrogated - Linear Fit of Measured

2 4

Heating Power (mWl

Heating Power (mW)

6

Figure 5. The interrogated and measured (by optical spectra analyzer) resonant wavelength of the microring sensor with the ring ofthe sensor under

different heating power. Inset shows the output intensity with different heating power applied on the microring interrogator

III. SUMMARY

In summary, both on-chip and off-chip wavelength­modulated sensor have been interrogated by a tunable silicon add-drop micro ring resonator. This interrogator is very compact and has a wide working range. These advantages have promised a great potential for our model being used in wavelength-modulated sensing applications.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (60977043), the National High Technology Research and Development Program of China (2012AAO 12203)_

REFERENCES

[I] G. Xiao, P. Zhao, F. Sun, Z. Lu, Z. Zhang and C. Grover, Opt. Lett., 29 (2004), pp. 2222-2224.

[2] G. Kim, H. Lee, C. Park, S. Lee, B. Lim, H. Bae and W. Lee, Opt. Exp., 18 (2010), pp. 22215-22221.