DE10134677B4 - Apparatus for spectral measurements by means of frequency combs - Google Patents

Abstract

Apparatus consisting of:
- A first radiation source for a frequency comb with mode spacing ν _r1
- a second radiation source for a frequency comb with mode spacing ν _r2 such that the difference | ν _r1 - ν _r2 | has a value Δν _r that is much smaller than ν _r1 and ν _r2 .
- Device to bring the radiation of at least one of the two radiation sources with a DUT in interaction.
- Device to superimpose the correspondingly changed radiation with the radiation of the other radiation source, such that interference takes place.
- Detector for the detection of interference.

Description

Often exists the need to adjust the spectral properties (absorption, permeability, Scattering) of a sample.

Usually For this purpose, sequential measurements are carried out in which the wavelength of a radiation source via a Spectral range is varied, and at the same time recorded the signal becomes.

Another method, the Fourier Transform Infrared Spectroscopy (FTIR), uses a radiation source with a continuous spectrum, the measurement requiring a certain integration time during which a path length is traversed in the interferometer (see for example the patent DE 19940981 C1 from Simon).

A parallel recording of a spectrum is a known technique, using a light source that emits a broad spectrum. When passing through the sample, the light is dispersed and is with recorded a photodiode / CCD array, from which one the transmission spectrum receives.

It is desirable an alternative method with high spectral resolution and fast measuring time too develop. About that In addition, laser radiation is desirable with which one can achieve a high sensitivity. Such a Method is described here.

Possible applications of this method are: microscopy, microspectral analysis, LIDAR.

in the Compared to the prior art describes the invention a method that is particularly applied in the spectral range can be found in the technically mature laser sources are available (Infrared to UV range). Also the important spectral range for optical data transmission is included. Furthermore can non-linear optical methods, such as parametric amplification and Self-phase modulation, the usable wavelength range to relatively simple Expand way. The spectral scope can be several octaves to reach. The invention is suitable in principle both for measuring of absorptive and dispersive properties, because the two are used frequency combs superimposed before and after the interaction with the sample and the beats are measured.

apart from the high spectral resolution speed, is the advantage of the invention in that that no dispersive components are needed. For measurements in the infrared one comes without expensive optical elements and photodiodes out.

The invention enables both simultaneous measurements of the properties of a sample over a broad spectral range, as well as sequential measurements at different frequency values. Particularly advantageous in the latter implementation by means of a laser is that no narrowband laser source is required for this, but this can be done with spectrally wide mode-locked lasers. Corresponding narrow-band lasers with a very large tuning range would be complex in production and operation. The selectivity in the frequency domain in the sequential measurement is not achieved directly but rather by the strict periodicity of the laser pulses. This aspect also distinguishes the invention from US patent US 42 97 035 by Bjorklund, which provides a narrowband laser source and describes a special detection technique.

In the patent DE 4437 575 C2 van der Weide and Keilmann describe a spectrometer with coherent and periodically pulsed radiation. When applied to wavelengths in the optical spectral range, however, the specified embodiment by means of the upconversion method of high complexity and hardly suitable in practice. Furthermore, it is assumed in the document of a common irradiation of the sample with two periodically pulsed radiation beams. However, this condition greatly restricts the information obtained from the sample.

In the patent DE 68927170 T2 Mitchell presents a measuring instrument that also contains harmonic comb generators but serves the purpose of measuring the temporal response function of a fluorescent sample, but not of spectral properties.

• 1.) Zwei modengekoppelte Laser erzeugen zwei unabhängige Frequenzkämme, FC1 und FC2, die, falls nötig, durch nichtlineare optische Methoden spektral verbreitert werden können.
• 2.) Der Servo stabilisiert die Repetitionsrate von FC2 auf einen Wert ν_r2 = ν_r1 – Δν_r, wobei Δν ≪ ν_r1, ν_r2 eine kleine Frequenz ist (z. B. 10 kHz). Auch stabilisiert er die relativen optischen Frequenzen derart, daß zwei bestimmte Frequenzen ν_1,0, ν_2,0 sehr nah beieinanderliegen, ν_2,0 = ν_1,0 + δ, wobei δ veränderbar ist.
• 3.) Der Photodetektor PD_ref zeichnet die Interferenzen zwischen den beiden Kämmen FC1, FC2 auf, siehe 2. Das RF-Spektrum des Signals von PD_ref enthält Information über die spektrale Transmission der Probe (s. Ansprüche 5., 6.). 3 zeigt, wie sich eine Reihe von Schwebungen zwischen jeweils einer Mode von FC1 und einer Mode von FC2 ergibt. Einzelne Schwebungen kommen z. B. bei folgenden RF-Frequenzen vor: ν2,0 – ν1,0 = δ ν2,0 + νr2 – (ν1,0 + νr1) = Δνr – δ ν2,0 + 2νr2 – (ν1,0 + 2νr1) = 2Δνr – δ, etc.

The basic principle of the arrangement resembles a vernier caliper acting in frequency space. An advantageous embodiment is in 1 shown (see claim 1).

• 1.) Two mode-locked lasers produce two independent frequency combs, FC1 and FC2, which, if necessary, can be spectrally broadened by non-linear optical methods.
• 2.) The servo stabilizes the repetition rate of FC2 to a value ν _r2 = ν _r1 -Δν _r , where Δν _r1 , ν _{r2 is} a small frequency (eg 10 kHz). Also, it stabilizes the relative optical frequencies such that two particular frequencies ν _1.0 , ν _{2.0 are} very close together, ν _2.0 = ν _1.0 + δ, where δ is variable.
• 3.) The photodetector PD _ref records the interference between the two combs FC1, FC2, see 2 , The RF spectrum of the signal from PD _ref contains information about the spectral transmission of the sample (see claims 5, 6). 3 shows how a series of beats results between each one mode of FC1 and one mode of FC2. Individual beats come z. At the following RF frequencies: ν 2.0 - ν 1.0 = δ ν 2.0 + ν r2 - (ν 1.0 + ν r1 ) = Δν r - δ ν 2.0 + 2ν r2 - (ν 1.0 + 2ν r1 ) = 2Δν r - δ, etc.

In order to avoid that a beat signal is generated at a certain beat frequency by different modes of FC2, one must set certain conditions. The two lowest order conditions are:

Example: For the election

gibt, die eine bestimmte Schwebungsfrequenz verursacht.These conditions (see claim 3) ensure that there is only a single mode in the field

which causes a certain beat frequency.

Of the Post of fashions outside this area can be suppressed By switching the relevant modes in the waves of FC1 and FC2 removed by optical filters.

ist. Für optische Wellen (ν_1,0 ≈ ν_2,0 ≈ 500 THz) ist das ein Wellenlängenbereich von ca. 50 nm.In order to maximize the above frequency range, it is useful to maximize the comb spacings, ν _r1 , ν _r2 , and to minimize the difference of comb spacings Δν _r . As an example, choose ν _r1 ≈ ν _r2 = 1 GHz, Δν _r = 10 kHz, so that the unique range

is. For optical waves (ν _1.0 ≈ ν _2.0 ≈ 500 THz) this is a wavelength range of about 50 nm.

was dem Transmissions-Koeffizienten der Probe bei der optischen Frequenz ν_2,N entspricht.The electric power of the photocurrent of PD _sig at the beat frequency | NΔν _r - δ | is by | E ₁ (ν _{1, N} ) E _{2, out} (ν _{2, N} ) | ² , where E ₁ (ν _{1, N} ), E _{2, out} (ν _{2, N} ) are the field strengths at the corresponding comb frequencies zen are ν _{1, N} = ν _1,0 + Nν _r1 , ν _{2, N} = ν _2,0 + Nν _r2 (N is a positive or negative integer). The ratio of the signal powers recorded by PD _sig and PD _ref at this beat frequency is proportional to

which corresponds to the transmission coefficient of the sample at the optical frequency ν _{2, N.}

• 1. Statt vollständige RF-Spektren mit den Photodetektoren PD_ref und PD_sig aufzuzeichnen, kann man diese Detektoren auch in einem schmalbandigen Modus betreiben, d. h. ihren Output bei einer bestimmten RF-Frequenz ν * / b, filtern. Dadurch erhält man Informationen über die Eigenschaften der Probe bei der optischen Frequenz ν * / 2 = ν_2,0 + N^* ν_r2, wobei N^* gegeben ist durch
  
  Wenn man den Kammabstand Δν_r verändert, aber bei einem festen Wert ν * / b, mißt, verändert man effektiv die optische Frequenz ν * / 2 und erhält sequentiell das Transmissionsspektrum.
• 2. Alternativ kann man das Photod →etektorsignal bei einer bestimmten RF-Frequenz ν * / b betrachten und die absolute Frequenz des Kamms FC2 in Schritten von Δν_r, d. h. ν_2,N → ν_2,N + nΔν_r, verändern (s. Anspruch 4). Das bedeutet eine Veränderung in Schritten von ν_r2 der optischen Frequenz derjenigen Mode von FC2, die das Signal bestimmt. Das Verändern des zweiten Kamms kann durchgeführt werden durch einen verstimmbaren akustooptischen Frequenzschieber, der nach dem ersten Strahlteiler (s. 1) eingefügt wird, oder dadurch, dass man δ in Schritten von Δν_r erhöht.

Other implementation possibilities are described below.

• 1. Instead of recording complete RF spectra with the photodetectors PD _ref and PD _sig , one can also operate these detectors in a narrowband mode, ie, filter their output at a given RF frequency ν * / b. This gives information about the properties of the sample at the optical frequency ν * / 2 = ν _2.0 + N ^* ν _r2 , where N ^{* is} given by
  
  If one changes the comb distance Δν _r , but measures at a fixed value ν * / b, one effectively changes the optical frequency ν * / 2 and receives the transmission spectrum sequentially.
• 2. Alternatively, one can consider the photod → etektorsignal at a certain RF frequency ν * / b and the absolute frequency of the comb FC2 in steps of Δν _r , ie ν _{2, N} → ν _{2, N} + nΔν _r , change (s Claim 4). This means a change in steps of ν _{r2 of} the optical frequency of the mode of FC2 that determines the signal. The altering of the second comb can be carried out by a tunable acousto-optical frequency shifter, which after the first beam splitter (see FIG. 1 ) or by increasing δ in steps of Δν _r .

The technical considerations are explained below.

The part of the servo off 1 Ensuring that ν _2.0 = ν _1.0 + δ can be realized by using an additional continuous wave laser of frequency ν _L , which lies within the optical spectrum of the two frequency combs ( 4 ). The beats between ν _L and FC1 and ν _L and FC2 are generated by overlapping the waves. The absolute frequencies of the combs are then stabilized by two servos 1,2 so that ν _1,0 = ν _L and ν _2,0 = ν _L + δ.

Claims (8)

Apparatus consisting of: - a first radiation source for a frequency comb with mode spacing ν _r1 - a second radiation source for a frequency comb with mode spacing ν _r2 such that the difference | ν _r1 - ν _r2 | has a value Δν _r that is much smaller than ν _r1 and ν _r2 . - Device to bring the radiation of at least one of the two radiation sources with a DUT in interaction. - Device to superimpose the correspondingly changed radiation with the radiation of the other radiation source, such that interference takes place. - Detector for the detection of interference. Apparatus of 1., such that the frequency difference δ of a mode of the first radiation source and a mode of the second radiation source has a value which differs from a multiple of half the mode spacing difference (Δν _r / 2). Apparatus of 2. such that said frequency difference δ is an odd multiple of Δν _r / 4. Apparatus according to one of the preceding claims, such that said frequency difference δ can be varied in steps of Δν _r and that the detector signal is spectrally selected. Apparatus according to any one of the preceding claims, such that the electrical bandwidth of the detector is substantially greater than Δν _r . Apparatus according to one of the preceding claims, additionally provided with a device for spectrally detecting the electrical detector signal disassemble. Apparatus according to one of the preceding claims, additionally provided with devices to phase lock the modes of each comb and the combs to be in phase with each other. Apparatus according to one of the preceding claims, additionally provided with a device to phase sensitive the electrical detector signal to measure.