We present several novel technologies for sensing millimeter-wave (mmW) radiation for imaging and spectroscopy based on photonic devices. Along these lines, in our high-sensitivity millimeter-wave (mmW) imaging system, which is based on optical upconversion, the power of mmW radiation is transferred to the sidebands on an optical carrier via an electro-optic (EO) modulator fed by a broadband horn antenna. The detection is realized by measuring the transferred optical power of the sidebands. The sensitivity of this detection system is primarily controlled by the conversion efficiency of the EO modulator at the desired mmW frequency (e.g. 95GHz). Thus, modulators are required that exhibit an ultra-broad bandwidth and small drive voltage. In this paper, we present the design, fabrication, and characteristics of LiNbO3 traveling-wave modulator for the mmW detection system. In a traveling-wave modulator, the bandwidth is limited by the mismatch between electrical and optical propagation constants. We have developed several techniques to finely tune the propagation constant of the mmWs in the modulator and have thereby eliminated this mismatch. Further bandwidth limitations for the modulator arise from losses in the electrode conductor, the substrate and buffer layer dielectrics, and coupling between the traveling-wave mode and the substrate modes. Modulator structures are described to reduce those losses without increasing the device driving voltage. The bandwidth and conversion limits of these structures are also discussed. The mmW detection pixels using the fabricated modulators were assembled, characterized, and analyzed. A high-sensitivity W-band detection system with a low noise-equivalent temperature difference (NETD) has been demonstrated. In addition, we present ongoing work to improve coupling millimeter-wave energy to the modulator at the W-band using techniques viable for packaged devices.
We report the design, fabrication, and characterization of high-speed LiNbO3 modulator for the millimeter-wave
(MMW) detection system at W band covering atmospheric window at 94 GHz. The LiNbO3 modulator is used to
convert the collected MMW power into optical frequency, and hence predominantly determines the system sensitivity.
The high sensitivity of detection requires the modulator a broad-band response and a small driving voltage. The ridged
traveling-wave structure has been used in the modulator design. The effects of velocity matching, impedance matching,
and MMW attenuations in this structure on the device's MMW conversion efficiency are investigated. A numerical
model has developed to optimize the device geometric parameters and the fabrication processes. The fabricated
modulator achieved the 3-dB optical bandwidth of 67 GHz and the conversion efficiency of ~0.7 W-1 at 94 GHz. The
detection pixel based on it has shown a high sensitivity with a noise equivalent temperature difference of ~6 K at a
refreshing rate of 30 Hz.
In this paper we present several novel photonic technologies for sensing millimeter-wave (MMW) radiation for the imaging and spectroscopy applications. Based on the optical up-conversion approach, our high-sensitivity MMW imaging system transfers the power of MMW radiation received from a broadband horn antenna to the sidebands on an optical carrier via an electrooptic (EO) modulator. The detection is realized by measuring the transferred optical power of the sidebands. The sensitivity of this detection system is primarily controlled by the conversion efficiency of the EO modulator
at the desired MMW frequency. In this paper, we present the design, fabrication, and characteristics of the ultra-broadband LiNbO3 traveling-wave modulator for the MMW detection system working at a frequency of 95 GHz. A numerical model based on the finite element analysis technique has developed to optimize the device geometric parameters and the fabrication processes. A modulation efficiency of ~0.9 W-1 at 95 GHz has been achieved for the optimized modulator, which corresponds to the half-wave voltages of 9 V and 18 V, at DC and 95 GHz, respectively. The detection pixel based on those modulators has shown a high sensibility with a noise equivalent temperature difference of ~17K at a refreshing rate of 30 Hz.
Recent efforts in our group towards the fabrication of sensors capable of detecting passive levels of millimeter-wave radiation have led to the development of an optically-based detector with sub-picowatt noise equivalent powers. This sensor is based on upconverting the received radiation into sidebands on an optical carrier using electro-optic modulation techniques and, subsequently, suppressing the remaining carrier energy. The noise equivalent power of such detectors is critically dependent on the ability of the electro-optic modulator to efficiently convert frequencies up to and exceeding 95 GHz onto the optical carrier while suppressing potential noise sources. In this paper, we discuss the specific device requirements generated by this unique potential application of high-frequency optical modulators. The effects of various modulator properties, such as half-wave voltage, frequency response, and maximum optical power density are discussed in the context of millimeter-wave detection capability. In addition, we present experimental efforts towards fabricating a passive millimeter-wave detector based on this approach, including efforts to develop an optimized optical modulator technology.
In this paper we present the modeling, design and fabrication of high-speed photonic modulators for use at high GHz, namely millimeter wave (MMW), frequencies based on the electro-optic materials, such as LiNbO3. To accurately design the traveling wave MMW modulators rigorous EM numerical tools are used to determine the propagation characteristics of both the optical and MMW waveguides. Extensive studies have been made to achieve an optimal design, which includes a close refractive index match between optical and MMW wave and a reduction of MMW propagation loss. The designed devices have been fabricated and tested with a modulation up to 135GHz.