I just read the 2015 excellent paper , "Tunable microwave photonic notch filter based on sliced broadband optical source".

Is it possible to downconvert a microwave photonic notch filter to a visible light photonic notch filter? What is the tuning speed of this microwave photonic filter?

[EDIT: August 4 2016 9:26 P.M Frank . The tuning speed of this analog microwave photonic filter is limited primarily by the response time of the balanced photodiode. For example, the Newport Nanosecond Photodetector, Silicon, 350-1000 nm, 0.8 mm Diameter, 8-32 / M4 Model: 1621 with URL, [https://www.newport.com/p/1621], has a rise time of 1 nanosecond.]

[EDIT: August 6 2016 6:00 P.M. Frank . Today's laser pointers are almost exclusively continuous wave)(CW) laser pointers . Continuous wave laser pointers do not need chirp compensation. Currently, pulsed lasers are used for high speed telecommunications because narrow pulse widths are needed. In the near future, it is estimated that CW laser pointers will be replaced by pulsed laser pointers which require chirp compensation similar to fiber Bragg grating (FBG) with variable delay line(VDL). Chirping is defined as a waveform with a continuously increasing or decreasing frequency spectra where frequency equals speed of light divided by wavelength.]

[EDIT August 7 2016 7:00 P.M. Frank There are two types of balanced photodetectors. Free space and Fiber, which is faster and smaller. The ThorLabs PBD430A Fiber model takes the difference between two signals , subtracting them across the spectrum for visible or infrared light . The PDB430A has two different frequency ranges 0 Hz to 400 MHz and 30Khz to 350 Mhz. ThorLabs told me last Friday that their PDB430A has a switching speed fast enought to respond to laser pointer strikes within 50 millisecond , thereby protecting the pilots from permanent damage.]

[EDIT August 7 2016 7:00 P.M switching frequency = speed of light (3.0 x 10^8 meters/second divided by wavelength 532 nanometers = 500 Terahertz switching time which is so fast one cannot see fluctuations of the electric field]

[EDIT August 8 2016 2:30 A.M.

This paper is very important because we wish to protect pilot's vision from increasingly sophisticated laser pointers on the ground 150 meters away aimed at the Boeing 747-787 cockpit windows. If we can detect the presence and wavelength of a coherent laser light in an incoherent background within 10 milliseconds of arrival , could I then activate a tunable band-reject filter based on this paper centered at the visible red, green or blue wavelengths?

In addition, I read with great interest in this paper that:

"The frequency omega2 can be tuned by the MZI without changing the filter shape according to Eq. (9). Another slicing way is to directly use the FPOP to shape the spectrum of the BOS. In this way, the interferometer is not needed in the approach, but a FPOP with high resolution is required."

In December 2009, Yuuki Watanabe and Toshiki Itagaki wrote in SPIE Journal of Biomedical Optics Volume 14 Issue6 JBO Letters 2009, "Real-time display on Fourier domain optical coherence tomography(OCT) system using a graphical processing unit" that a display rate of 27.9 frames per second for processed images(2048 FFT size X 1000 lateral A-scans) is achieved in our OCT system using a line scan CCD camera operated at 27.9 kHz using the C and C++ programming language".

Given today's embedded microprocessors, could we boost the FDOP display rate of 27.9 frames per second to 500 frames per second to meet Boeing and Airbus passenger plane pilot's expectations?

Furthermore , I read with great interest in the paper that:

"In this paper, two filters are necessary. One filter is used to filter the BOS; the other one is used to split the single laser from the BOS. In order to quickly verify the principle, we use two waveshapers in the experiment. It results in a high system cost. Moreover, a way to reduce the cost is to use fiber Bragg gratings (FBG) with optical circulators to work as the filters. Take the second filter as an example, we can send the light into the FBG via an optical circulator. When the Bragg wavelength is just equal to the laser wavelength, the reflected light only includes the laser light and the transmitted light only contains the BOS part."

How could I estimate the performance loss associated with replacing the waveshaper with fiber Bragg gratings and optical circulator?

This paper is a very nice example of out-of-the box engineering because it has no moving elements or electro-optic Pockels or Kerr effect. Rather, the authors subtract a single frequency source from an allpass optical filter producing a band-reject filter.

Fig. 1 The schematic diagram. SSB: Single-side-band modulator. The schematic diagram of the proposed microwave photonic filter (MPF) is illustrated in the above figure. The single-side-band modulator is a fancy name for a microwave frequency mixer. The optical power and optical angle frequency of the laser source is 𝑃 𝑜 and 𝜔 𝑜 , respectively. The spectrum-sliced broadband optical source (BOS) is first confined within the optical angle frequency range between 𝜔 1 and 𝜔 2 (𝜔 1 <𝜔 2 ) via the Fourier domain optical processor (FDOP), and then sliced via a Mache-Zehnder interferometer (MZI) which is constructed by two 50/50 optical couplers and a variable delay line (VDL). The FDOP works as a programmable optical filter. The single-frequency light and the broadband light are coupled together and then single-side-band modulated to carry the radio frequency (RF) signal. The modulated light is fed into a dispersion fiber, and then launched into a wavelength division multiplexer (WDM). The WDM splits the light into two parts: Port B only includes the laser part; Port C only includes the BOS part. A balanced photodetector (BPD) is employed to detect the two parts, respectively.

Please correct me if I have stated anything incorrect. The use of equations is appreciated.

Any help is greatly appreciated.

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  • $\begingroup$ Frank - comments are not for extended discussion. $\endgroup$
    – user16
    Aug 6 '16 at 12:19
  • $\begingroup$ @GlenH7, Please tell me why I lost 4 points of reputation in the last 24 hours. I plan to add some information from ThorLabs later tonight in the body of the question. $\endgroup$
    – Frank
    Aug 7 '16 at 0:20
  • $\begingroup$ Do you understand the difference between a filter and a tuned oscillator? $\endgroup$ Aug 8 '16 at 13:03
  • $\begingroup$ @Carl Witthoft, I will respond to your question tonight August 8 2016. Thank you. $\endgroup$
    – Frank
    Aug 8 '16 at 17:46
  • $\begingroup$ @Carl Witthoft, Quoting from David Pozar's Microwave Engineering, "An oscillator is a nonlinear circuit that converts DC power to an AC waveform. Most RF oscillators provide sinusoidal outputs which minimize undesired harmonics and noise sidebands ". In contrast, A filter is a two-port network used to control the frequency response at a certain point by providing transmission within the passband and attenuation in the stopband. May I meet you when I fly to Boston this weekend or next weekend? Thank you. $\endgroup$
    – Frank
    Aug 9 '16 at 1:17

I discovered this morning that one can adapt the Tsinghua University professor's novel design and analysis microwave photonic notch filter to a visible light photonic notch filter by applying the excellent Princeton University paper titled "High frequency modulation capabilities and quasi single-sideband emission from a quantum cascade laser" with URL [https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-22-19-23439&id=301440]. Here is an excerpt from that 2014 Princeton University Department of Electrical Engineering paper;


Both intensity- (IM) and frequency-modulation (FM) behavior of a directly modulated quantum cascade laser (QCL) are measured from 300 Hz to 1.7 GHz. Quantitative measurements of tuning coefficients has been performed and the transition from thermal- to electronic-tuning is clearly observed. A very specific FM behavior of QCLs has been identified which allows for optical quasi single sideband (SSB) modulation through current injection and has not been observed in directly modulated semiconductor lasers before. This predestines QCLs in applications where SSB is required, such as telecommunication or high speed spectroscopy. The experimental procedure and theoretical modeling for data extraction is discussed.

© 2014 Optical Society of America

  1. Introduction

Since the first demonstration of the single-mode, room temperature and continuous wave operating quantum cascade laser (QCL) less than a decade ago [1], a number of applications were triggered that use their advantages as monolithic mid-infrared (mid-IR) semiconductor lasers. One of the most significant applications is the spectroscopic molecular sensing, which can be several orders of magnitude more sensitive than in the near infrared. These monolithic, tunable mid-IR lasers allow development of both compact and highly sensitive trace-gas detection systems. Tunable laser spectroscopy with semiconductor laser sources inherently relies on the wavelength tuning capability through variation of laser injection current. However, since this process is not arbitrarily fast this imposes some limitations on the applications.

There were only a few reports in the literature on the experimental results of the (small-signal) modulation response of directly modulated QCLs. Examples include [2] (intensity modulation, IM) [3], (frequency modulation (FM) up to 100 kHz), and [4] (only THz QCLs). Significantly more extensive are reports on the FM response in diode lasers [5,6].

Main differences between QCLs and conventional interband DFB diode lasers (e.g., those operating at telecom wavelengths) are expected from the thermal implications of significantly thicker active regions and the fundamentally different carrier dynamics in QCLs. The latter is due to the ultrashort intersubband carrier lifetimes, according to which QCLs can have relaxation frequencies in the 100 GHz range. This has been shown theoretically by Shore et al. [7,8]. Hence, it is very interesting to have experimental studies in the high frequency regime. Furthermore, the electronic tuning (at f >>10MHz) could be quantified.

In this paper the FM to IM ratio for a CW-DFB-QCL emitting at ~9 µm is presented for a wide range of modulation frequencies up to 1.7 GHz. The variation of the FM-IM ratio with bias current and its influence on the optical spectrum of the laser radiation is analyzed.

  1. Optical frequency tuning mechanisms and emitted spectrum

The tuning of optical frequency of a semiconductor laser through injection current is mainly caused by two physical effects [9,6]: firstly the Joule heating of the active region results in thermally induced changes of the refractive index, which causes frequency tuning of the emitted laser radiation, and secondly the refractive index variation from the electro-optic light-carrier interaction also affects the laser output frequency. Since heat conduction is an inert process, the low frequency behavior is mostly dominated by thermal tuning, whereas the electronic tuning is governed by the carrier dynamics and for diode lasers dominates in the MHz and GHz range.

[Frank's EDIT August 9 2016 2:01A.M The tuning speed of this analog microwave photonic filter is limited primarily by the response time of the balanced photodiode. For example, the Newport Nanosecond Photodetector, Silicon, 350-1000 nm, 0.8 mm Diameter, 8-32 / M4 Model: 1621 with URL, [https://www.newport.com/p/1621], has a rise time of 1 nanosecond.]

Please let me know if I have made any mistakes.

  • 1
    $\begingroup$ It's unclear what you actually want to know, and what problem you are trying to solve. $\endgroup$ Aug 8 '16 at 13:04
  • $\begingroup$ @Carl Witthoft, I agree with you. There is misuse of the term downconversion which I wil try to correct tonight. Thanks. $\endgroup$
    – Frank
    Aug 8 '16 at 17:48
  • $\begingroup$ @Carl Witthoft, I am sorry that I was not clear originally in this question. May I ask you to consider the following post which has a better problem statement? physics.stackexchange.com/questions/273005/… titled ,"Can we tune the Mach-Zehnder modulator or equally mix it with a pure RGB laser signal with a broadband optical source at the same efficiency?" $\endgroup$
    – Frank
    Aug 9 '16 at 5:52

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