## 1 Introduction
It turned out that the absorption coefficient of the order of 10\({}^{-6}\) cm\({}^{-1}\) in the optical elements of the scheme can destroy the mode of highly efficient (~40%) power conversion. The TPCI scheme is a modification of the PCI method and by now demonstrates the highest experimentally achieved sensitivity of measuring the absorption coefficient in quartz glasses: \(\leq\)10\({}^{-6}\) cm\({}^{-1}\) with a signal-to-noise ratio \(\geq\)50 [11]. The achievement of such parameters for ultra-low absorption measurements in this scheme is associated with several reasons [12; 13; 11]. 1. The application of electronic modulation of the heating laser radiation intensity. Meanwhile, in most other schemes a mechanical chopper is employed. The main advantage of the electronic modulation is the possibility of using methods of digital processing of measured signal waveforms obtained with the help of an 8-bit analog-to-digital converter (ADC). At such processing of the measured signal with the number of realizations \(\sim\)10\({}^{4}\) in the process of its averaging by the 64-bit processor there is an increase in the bit rate of the averaged signal [12; 13]. It allows research to be carried out in weakly absorbing samples and for the measurement of weak signals with values within one ADC bit in conditions when technical noise values correspond to eight ADC bits. It also makes it possible to determine and compensate for the influence of different parasitic effects, thus effectively increasing the sensitivity of measurements. 2. Temporal resolution of the shape of the time-varying component of the probe beam power. This resolution allows the observed absorption-based signal to be separated from distortions caused by refractive index nonlinearity in the heating radiation field and by heat diffusion from the heated area [11; 12; 13]. 3. Carrying out theoretical calculations using simple analytical formulas relating the level of the observed signals to the value of the absorption coefficient [11]. This allows a simple calibration of measurements using glasses with known absorption coefficients, which is especially relevant for crystal measurements [12; 13]. 4. Clear criteria for tuning of the scheme determined by the coaxiality of the beams of heating and probe radiation. This configuration, in addition to the theoretical calculations mentioned in point 3, allows us to obtain reproducible results of the absorption coefficient values when changing samples and, thus, to obtain calibration coefficients with a small spread of possible values. It should be mentioned that our earlier estimates of the sensitivity of the TPCI scheme [12], based on the measurement of the noise component of the large signal in Suprasil 311 glasses with a relatively large absorption coefficient (2.6 \(\times\) 10\({}^{-6}\) cm\({}^{-1}\)), turned out to be too optimistic. This is due to the fact that when measured in samples with the absorption coefficient at the level of \(\sim\)10\({}^{-7}\) cm\({}^{-1}\), other physical effects that mask the signal associated with absorption begin to influence to the desired signal. In particular, this is the absorption of ambient air [14], the elimination of which is the subject of this work. As shown below, when this effect is eliminated, other disturbances may arise that distort the signal already at lower absorption levels \(\leq\)10\({}^{-8}\) cm\({}^{-1}\). This remark also refers to theoretical estimates of the limiting sensitivity of the methods (given, e.g., by the authors [16; 17]), which may be encouraging, but which have no sense in relation to practical purposes. This paper is a continuation of [14; 15], in which measurements of the absorption coefficient at a wavelength of 1071 nm were carried out in synthetic crystalline quartz under the influence of ambient air absorption. The measurements were carried out using a TPCI scheme in which the optical part was modified compared to the original version described in [11].